The  Lunar  "Crater"  Copernicus 

Photographed  from  Nasmyth  and  Carpenter's  plaster-of-paris  model 

of  the  moon. 

In  this  model  the  topography  of  the  moon  is  faithfully  represented 
seen  with  powerful  telescopes. 


Astronomy    in    a 
Nutshell- 

The   Chief   Facts   and    Principles 

Explained  in  Popular  Language 

for  the  General  Reader 

and  for  Schools 


By 

Garrett  P.  Serviss 


With  47  Illustrations 


G.  P.  Putnam's  Sons 

New  York  and  London 
fmfcfterbocfcer    press 
1912 


COPYRIGHT,  1912 

BY 
GARRETT  P.  SERVISS 


Ube  ttmcfcerbocfcer  press,  Hew  jQorb 


PREFACE 

How  many  thousands  of  educated  people, 
trained  in  the  best  schools,  or  even  graduates 
of  the  great  universities,  have  made  the  con- 
fession: "  I  never  got  a  grip  on  astronomy  in 
my  student  days.  They  didn't  make  it 
either  plain  or  interesting  to  me ;  and  now  I 
am  sorry  for  it." 

The  purpose  of  the  writer  of  this  book  is  to 
supply  the  need  of  such  persons,  either  in 
school,  or  at  home,  after  school-days  are 
ended.  He  does  not  address  himself  to  special 
students  of  the  subject — although  they,  too, 
may  find  the  book  useful  at  the  beginning — 
but  to  that  vast,  intelligent  public  for  whom 
astronomy  is,  more  or  less,  a  "  mystical  mid- 
land," from  which,  occasionally,  fascinating 
news  comes  to  their  ears.  The  ordinary 
text-book  is  too  overladen  with  technical 
details,  and  too  summary  in  its  treatment  of 
the  general  subject,  to  catch  and  hold  the 
attention  of  those  who  have  no  special  pre- 
liminary interest  in  astronomy.  The  aim 
here  is  to  tell  all  that  really  needs  to  be  told, 


242443 


iv  Preface 

and  no  more,  and  to  put  it  as  perspicuously, 
compactly,  and  interestingly,  as  possible. 
For  that  reason  the  book  is  called  a ' '  nutshell. " 


The  author  has  been  sparing  in  the  use  of 
diagrams,  because  he  believes  that,  in  many 
cases,  they  have  been  over-pressed.  There  is 
a  tendency  to  try  to  represent  everything  to 
the  eye.  This  is  well  to  a  certain  extent,  but 
there  is  danger  that  by  pursuing  this  method 
too  far  the  power  of  mental  comprehension  will 
be  weakened.  After  all,  it  is  only  by  an 
intelligent  use  of  the  imagination  that  progress 
can  be  made  in  such  a  science  as  astronomy. 
The  reader  is  urged  to  make  a  serious  effort 
to  understand  what  is  said  in  the  text,  and  to 
picture  it  in  his  mind's  eye,  before  referring 
to  the  diagrams.  After  he  has  thus  presented 
the  subject  to  his  imagination,  he  may  refer 
to  the  illustrations,  and  correct  with  their  aid 
any  misapprehension.  For  this  reason  the 
cuts,  with  their  descriptions,  have  been  made 
independent  of  the  regular  text,  although 
they  are  placed  in  their  proper  connections 
throughout  the  book. 

G.  P.  S. 
April,  1912. 


CONTENTS 

PAGES 

PART  I — THE  CELESTIAL  SPHERE. 

Definition  of  Astronomy —  Fundamental  Law  of  the 
Stars — Relations  of  the  Earth  to  the  Universe — 
Ordinary  Appearance  of  the  Sky — The  Horizon, 
the  Zenith,  and  the  Meridian — Locating  the 
Stars — Altitude  and  Azimuth — Circular  or  An- 
gular Measure — Altitude  Circles  and  Vertical 
Circles — The  Apparent  Motion  of  the  Heavens 
— The  North  Star  and  Phenomena  Connected 
with  it — Revolution  of  the  Stars  round  the  Pole 
— Locating  the  Stars  on  the  Celestial  Sphere 
— Astronomical  Equivalents  of  Latitude  and 
Longitude — Parallels  of  Declination  and  Hour 
Circles — "The  Greenwich  of  the  Sky" — Effects 
Produced  by  Changing  the  Observer's  Place  on 
the  Earth— The  Parallel  Sphere,  the  Right 
Sphere,  and  the  Oblique  Sphere — The  Astro- 
nomical Clock — The  Ecliptic — Apparent  Annual 
Revolution  of  the  Sun  round  the  Earth — In- 
clination of  the  Ecliptic  to  the  Equator:  its 
Cause  and  its  Effects — The  Equinoxes — Import- 
ance of  the  Vernal  Equinox — The  Equinoctial 
Colure — The  Solstices — Poles  of  the  Ecliptic — 
Celestial  Latitude  and  Longitude — The  Zodiac 
— The  Precession  of  the  Equinoxes:  its  Cause 
and  Effects — Revolution  of  the  Celestial  Poles — 
Past  and  Future  Pole  Stars  ....  3-64 


vi  Contents 


PART  II— THE  EARTH. 

Nature,  Shape,  and  Size  of  the  Earth — The  Polar 
Compression  and  Equatorial  Protuberance,  and 
their  Cause — The  Attraction  of  Gravitation — 
The  Mass  of  the  Earth :  how  Found — How  the 
Earth  Holds  the  Moon,  and  the  Sun  the  Planets 
— The  Tides — How  the  Moon  and  Sun  Produce 
Tides— Spring  Tides  and  Neap  Tides— The 
Atmosphere — The  Law  and  Effects  of  Refraction 
— Dip  of  the  Horizon — The  Aberration  of  Light 
— Time:  how  Measured — Sidereal,  Apparent 
Solar,  and  Mean  Solar  Time — The  Clock  and 
the  Sun — Day  and  Night — Where  the  Days 
Begin — The  Seasons — Effects  of  the  Varying 
Declination  of  the  Sun — Polar  and  Equatorial 
Day  and  Night — The  Tropics  and  the  Polar 
Circles — Inequality  of  Length  of  the  Seasons — 
When  the  Seasons  in  the  Two  Hemispheres  will 
be  Reversed— The  Calendar,  the  Year,  and  the 
Month — Reformations  of  the  Calendar — Differ- 
ent Measures  of  the  Month  .  .  .  67-123 


PART  III — THE  SOLAR  SYSTEM. 

The  Sun — Distance,  Size,  and  Condition  of  the 
Sun — Temperature  of  the  Sun — Solar  Heat  on 
the  Earth,  and  its  Mechanical  Equivalent — 
Peculiar  Rotation  of  the  Sun — Sun-spots,  their 
Appearance  and  Probable  Cause — Faculae — The 
Photosphere  —  Solar  Prominences  —  Explosive 
Prominences — The  Solar  Corona — Parallax,  and 
the  Measure  of  Distances — Spectroscopic  Analy- 
sis— How  the  Elements  in  the  Sun  Reveal  their 
Presence — List  of  the  Principal  Solar  Elements 


Contents  vii 


— The  Moon — Origin  of  the  Moon — Appearance 
of  its  Surface — Gravity  on  the  Moon — The 
Phases  of  the  Moon — Causes  of  the  Absence  of  a 
Lunar  Atmosphere — Eclipses — How  the  Moon 
Causes  Eclipses  of  the  Sun — The  Laws  Govern- 
ing Eclipses — The  Shadow  during  a  Solar  Eclipse 
— Eclipses  of  the  Moon — Number  of  Eclipses  in 
a  Year — The  Saros — The  Planets— Kepler's 
Laws  of  Planetary  Motion — Mercury — Venus — 
Mars,  and  its  So-called  Canals — Theories  about 
Mars — Jupiter,  its  Belts  and  its  Satellites — The 
"Great  Red  Spot" — Saturn,  its  Rings  and  its 
Satellites — Composition  of  the  Rings — Uranus 
and  Neptune — Comets,  and  the  Laws  of  their 
Motion — Composition  of  Comets — The  Pressure 
of  Light  and  its  Connection  with  Comets'  Tails 
— Breaking  up  of  Comets — Meteors  and  their 
Relations  to  Comets — The  November  Meteors 
and  Other  Celebrated  Showers — Meteorites  or 
Bolides  which  Fall  upon  the  Earth— The  Question 
of  their  Origin  .....  127-215 

PART  IV— THE  FIXED  STARS. 

Division  of  the  Stars  into  Magnitudes — Division  of 
the  Stars  according  to  their  Spectra — Stars 
Larger  and  Smaller  than  the  Sun — The  Distances 
of  the  Stars — Variable  Stars — Double  and 
Binary  Stars — Spectroscopic  Binaries  and  how 
they  are  Discovered — Proper  Motions  ©f  the 
Stars — Number  of  the  Stars — New,  or  Tempo- 
rary Stars— The  Milky  Way — The  Nebulae — 
The  Two  Kinds  of  Nebulae— Spiral  Nebulas— 
The  Nebular  Hypothesis — Applications  of  Pho- 
tography to  Stars  and  Nebulae — The  Constel- 


viii  Contents 

PAGES 

lations — How  to  Learn  the  Constellations — 
Their  Antiquity — Description  of  the  Principal 
Constellations  Visible  from  the  Northern  Hemi- 
sphere at  Various  Times  of  the  Year  .  219-257 

INDEX          .         .        .         .         .        .         .     259 


ILLUSTRATIONS 

PAGE 

THE  LUNAR  "CRATER"  COPERNICUS         Frontispiece 

PHOTOGRAPH  OF  SOUTH  POLAR  REGION  OF  THE  MOON  8 

THE  MOON  NEAR  THE  "CRATER"  TYCHO       .         .  20 
DRAWING  OF  JUPITER  .         .         .         .         .         .28 

DRAWING  OF  JUPITER  ......  28 

JUPITER      ........  38 

SATURN       ........  46 

SATURN 46 

THE  MILKY  WAY  ABOUT  CHI  CYGNI*  ...  58 

THE  GREAT  SOUTHERN  STAR-CLUSTER  IN  CENTAURI  64 

PHOTOGRAPH  OF  A  GROUP  OF  SUN-SPOTS      .         .  76 

POLAR  STREAMERS  OF  THE  SUN,  ECLIPSE  OF   1889  88 

SOLAR  CORONA  AT  THE  ECLIPSE  OF  1871          .         .  88 

MOREHOUSE'S  COMET,  OCTOBER  15,  1908        .         .  96 

MOREHOUSE'S  COMET,  NOVEMBER  15,  1908     .         .  96 

HEAD  OF  THE  GREAT  COMET  OF  1861      .         .         .  104 

HALLEY'S  COMET,  MAY  5,  1910     ....  104 
ix 


x  Illustrations 

PACK 

THE  SIX-TAILED  COMET  OF  1744   .         .  .  .112 

SPIRAL  NEBULA  IN  URSA  MAJOR  (M  101)  .  .     124 

THE  WHIRLPOOL  NEBULA  IN  CANES  VENATICI  .     124 

TRESS  NEBULA  (N.  G.  C.  6992)  IN  CYGNUS  .  .     132 

THE  GREAT  ANDROMEDA  NEBULA          .  .  .     140 

SPIRAL  NEBULA  IN  CEPHEUS  (H.  IV.  76)  .  ,     154 

NEBULOUS  GROUNDWORK  IN  TAURUS     .  .  .     154 

NEBULA  IN  SAGITTARIUS  (M.  8)     .         .  .  .162 

THE  GREAT  NEBULA  IN  ORION      .         .  .  .180 

PHOTOGRAPHS  OF  MARS         .         .        r  .     200 

SCHIAPARRELLI'S   CHART  OF  MARTIAN   "  CANALS "  .       22O 

ILLUSTRATIONS  IN  THE  TEXT 

THE  RATIONAL  AND  THE  SENSIBLE  HORIZON        .       12 
ALTITUDE  AND  AZIMUTH        ..        .         .         .         .14 

RIGHT  ASCENSION  AND  DECLINATION      .  .       35 

THE    ECLIPTIC    AND    CELESTIAL    LATITUDE     AND 

LONGITUDE  .  .  51 

How  THE  EARTH  CONTROLS  THE  MOON  .       75 

THE  TIDAL  FORCE  OF  THE  MOON  .         .  -79 

REFRACTION        . 85 

DIP  OF  THE  HORIZON    .         .         .         .         .         .87 

SIDEREAL  AND  SOLAR  TIME 93 

I 


Illustrations  xi 

PAGE 

THE  CHANGE  OF  DAY 101 

THE  SEASONS       .......     107 

PARALLAX  OF  THE  MOON 139 

PARALLAX  OF  THE  SUN  FROM  TRANSIT  OF  VENUS       .     141 
SPECTRUM  ANALYSIS     .         .         .         .         .         .147 

THE  PHASES  OF  THE  MOON  .....     160 

ORBITS  OF  MARS  AND  THE  EARTH  .         .         .183 

ELLIPSE,  PARABOLA,  AND  HYPERBOLA    .         .         .     203 
THE  NORTH  CIRCUMPOLAR  STARS  ....     244 

KEY  TO  NORTH  CIRCUMPOLAR  STARS      .         .        .    245 


PART  I. 

THE  CELESTIAL  SPHERED 


PART  I. 

THE  CELESTIAL  SPHERE. 

i.  Definition  of  Astronomy.  Astronomy 
has  to  do  with  the  earth,  sun,  moon,  planets, 
comets,  meteors,  stars,  and  nebulae;  in  other 
words,  with  the  universe,  or  "the  aggregate 
of  existing  things."  It  is  the  most  ancient 
of  all  sciences.  The  derivation  of  the  name 
from  two  Greek  words,  aster,  "star,"  and 
nomos,  "  law, "  indicates  its  nature.  It  deals 
with  the  law  of  the  stars — the  word  "star" 
being  understood,  in  its  widest  signification, 
as  including  every  heavenly  body  of  what- 
ever kind.  The  earth  itself  is  such  a  body. 
Since  we  happen  to  live  on  the  earth,  it 
becomes  our  standpoint  in  space,  from  which 
we  look  out  at  the  others.  But,  if  we  lived 
on  some  other  planet,  we  would  see  the  earth 
as  a  distant  body  in  the  sky,  just  as  we  now 
see  Jupiter  or  Mars. 

Astronomy  teaches  us  that  everything  in 

3 


4  THe  Celestial  SpHere 

the  universe,  from  the  sun  and  the  moon 
to  the  most  remote  star  or  the  most  extra- 
ordinary nebula,  is  related  to  the  earth.  All 
are  made  of  similar  elementary  substances 
and  all  obey  similar  physical  laws.  The 
same  substance  which  is  a  solid  upon  the 
earth  may  be  a  gas  or  a  vapour  in  the  sun, 
but  that  does  not  alter  its  essential  nature. 
Iron  appears  in  the  sun  in  the  form  of  a  hot 
vapour,  but  fundamentally  it  is  the  same 
substance  which  exists  on  the  earth  as  a 
hard,  tough,  and  heavy  metal.  Its  different 
states  depend  upon  the  temperature  to 
which  it  is  subjected.  The  earth  is  a  cool 
body,  while  the  sun  is  an  intensely  hot 
one;  consequently  iron  is  solid  on  the  earth 
and  vaporous  in  the  sun,  just  as  in  winter 
water  is  solid  ice  on  the  surface  of  a  pond 
and  steamy  vapour  over  the  boiler  in  the 
kitchen.  Even  on  the  earth  we  can  make 
iron  liquid  in  a  blast  furnace,  and  with  the 
still  greater  temperatures  obtainable  in  a 
laboratory  we  can  turn  it  into  vapour, 
thus  reducing  it  to  something  like  the  state 
in  which  it  regularly  exists  in  the  sun. 

This  fact,  that  the  entire  universe  is  made 
up  of  similar  substances,  differing  only  in 
state  according  to  the  local  circumstances 


Situation  of  E-artK  in  Heavens     5 

affecting  them,  is  the  greatest  thing  that 
astronomy  has  to  tell  us.  It  may  be  regarded 
as  the  fundamental  law  of  the  stars. 

2.  The  Situation  of  the  Earth  in  the 
Heavens.  One  of  the  greatest  triumphs 
of  human  intelligence  is  the  discovery  of  the 
real  place  which  the  earth  occupies  in  the 
universe.  This  discovery  has  been  made  in 
spite  of  the  most  deceptive  appearances. 
If  we  accepted  the  sole  evidence  of  our  eyes, 
as  men  once  did,  we  could  only  conclude  that 
the  earth  was  the  centre  of  the  universe. 
In  the  daytime  we  see  the  sun  apparently 
moving  through  the  sky  from  east  to  west, 
as  if  it  were  travelling  in  a  circle  round  the 
earth,  overhead  by  day  and  underfoot  at 
night.  In  the  night-time,  we  see  the  stars 
apparently  travelling  round  the  earth  in  the 
same  way  as  the  sun.  The  fact  is,  that  all 
of  them  are  virtually  motionless  with  regard 
to  the  earth,  and  their  apparent  movements 
through  the  sky  are  produced  by  the  earth's 
rotation  on  its  axis.  The  earth  turns  round 
on  itself  once  every  twenty-four  hours, 
like  a  spinning  ball.  Imagine  a  fly  on  a 
rotating  school  globe ;  the  whole  room  would 
appear  to  the  fly  to  be  revolving  round  it 
as  the  heavens  appear  to  revolve  round  the 


6  THe  Celestial  SpHere 

earth.  It  would  have  to  be  a  very  intelligent 
insect  to  correct  the  deceptive  evidence  of 
its  eyes. 

The  actual  facts,  revealed  by  many  cen- 
turies of  observation  and  reasoning,  are  that 
the  earth  is  a  rotating  globe,  turning  once 
on  its  axis  every  twenty-four  hours  and 
revolving  once  round  the  sun  every  three 
hundred  and  sixty-five  days.  The  sun  is 
also  a  globe,  1,300,000  times  larger  than  the 
earth,  but  so  hot  that  it  glows  with  intense 
brilliance,  while  the  substances  of  which 
it  consists  are  kept  in  a  gaseous  or  vaporous 
state.  Besides  the  earth  there  are  seven 
other  principal  globes,  or  planets,  which 
revolve  round  the  sun,  at  various  distances 
and  in  various  periods,  and,  in  addition 
to  these,  there  are  hundreds  of  smaller 
bodies,  called  asteroids  or  small  planets. 
There  are  also  many  singular  bodies  called 
comets,  and  swarms  of  still  smaller  ones 
called  meteors,  which  likewise  revolve  round 
the  sun. 

The  earth  and  the  other  bodies  of  which 
we  have  just  spoken  are  not  only  cooler 
than  the  sun,  but  most  of  them  are  in  a  solid 
state  and  do  not  shine  with  light  of  their 
own.  The  sun  furnishes  both  heat  and 


Situation  of  EartH  in  Heavens       7 

light  to  the  smaller  and  cooler  bodies  revolv- 
ing round  it.  In  fact,  the  sun  is  simply  a 
star,  resembling  the  thousands  of  other 
stars  which  surround  us  in  the  sky,  and 
its  apparent  superiority  to  them  is  due  only 
to  the  fact  that  it  is  relatively  near-by  while 
they  are  far  away.  It  is  probable  that  all, 
or  most,  of  the  stars  also  have  planets, 
comets,  and  meteors  revolving  round  them, 
but  invisible  owing  to  their  immense  distance. 
The  "paths"  in  which  the  earth  and  the 
other  planets  and  bodies  travel  in  their 
revolutions  round  the  sun  are  called  their 
orbits.  These  orbits  are  all  elliptical  in 
shape,  but  those  of  the  earth  and  the  other 
large  planets  are  not  very  different  from 
circles.  Some  of  the  asteroids,  and  all  of 
the  comets,  however,  travel  in  elliptical 
orbits  of  considerable  eccentricity,  i.  e., 
which  differ  markedly  from  circles.  The 
orbit  of  the  earth  differs  so  slightly  from  a 
circle  that  the  eccentricity  amounts  to  only 
about  one-sixtieth.  The  distance  of  the  earth 
from  the  sun  being,  on  the  average,  93,000,000 
miles,  the  eccentricity  of  its  orbit  causes  it 
to  approach  to  within  about  91,500,000 
miles  in  winter  (of  the  northern  hemisphere) 
and  to  recede  to  about  94,500,000  miles 


8  THe  Celestial  Sphere 

in  summer.  The  point  in  its  orbit  where 
the  earth  is  nearest  the  sun  is  called  peri- 
helion, and  the  point  where  it  is  farthest 
from  the  sun,  aphelion.  /  The  earth  is  at 
perihelion  about  _J  •  i..  i ,  and  at  aphelion 
about  Jttiy-4- 

Now,  in  order  to  make  a  general  picture  in 
the  mind  of  the  earth's  situation,  let  the 
reader  suppose  himself  to  be  placed  out  in 
space  as  far  from  the  sun  as  from  the  other 
stars.  Then,  if  he  could  see  it,  he  would 
observe  the  earth  as  a  little  speck,  shining 
like  a  mote  in  the  sunlight,  and  circling  in 
its  orbit  close  around  the  sun.*  The  universe 
would  appear  to  him  to  be  somewhat  like  an 
immense  spherical  room  filled  with  scattered 
electric-light  bulbs,  suspended  above,  below, 
and  all  around  him,  each  of  these  bulbs  repre- 
senting a  sun,  and  if  there  were  minute  insects 
flying  around  each  light,  these  insects  would 
represent  the  planets  belonging  to  the 
various  suns.  One  of  the  glowing  bulbs 
among  the  multitude  would  stand  for  our 
sun,  and  one  of  the  insects  circling  round 
it  would  be  the  earth.  t 

We  have  already  remarked  that  the 
rotation  of  the  earth  on  its  axis  causes  all 
the  other  heavenly  bodies  to  appear  to  revolve 


Photograph  of  the  South  Polar  Region  of  the  Moon 

Made  by  G.  W.  Ritchey  with  the  forty-inch  refractor  of  the  Yerkes 
Observatory. 


Horizon,  ZenitH,  and   Meridian     9 

round  it  once  every  twenty-four  hours,  and 
we  must  now  add  that  the  earth's  revolution 
round  the  sun  causes  the  same  bodies  to 
appear  to  make  another,  slower  revolution 
round  it  once  every  year.  This  introduces 
a  complication  of  apparent  motions  which  it 
is  the  business  of  astronomy  to  deal  with, 
and  which  we  shall  endeavour  to  explain. 

3.  The  Horizon,  the  Zenith,  and  the 
Meridian.  First,  let  us  consider  what  is  the 
ordinary  appearance  of  the  sky.  When  we 
go  out  of  doors  on  a  clear  night  we  see  the 
heavens  in  the  shape  of  a  great  dome  arched 
above  us  and  filled  with  stars.  What  we 
thus  see  is  one  half  of  the  spherical  shell 
of  the  heavens  which  surrounds  us  on  all 
sides,  the  earth  being  apparently  placed  at 
its  centre.  The  other  half  is  concealed  from 
our  sight  behind,  or  below,  the  earth.  This 
spherical  shell,  of  which  only  one  half  is 
visible  to  us  at  a  time,  is  called  the  celestial 
sphere.  Now,  the  surface  of  the  earth 
seems  to  us  (for  this  is  another  of  the 
deceptive  appearances  which  astronomy  has 
to  correct)  to  be  a  vast  flat  expanse,  whose 
level  is  broken  by  hills  and  mountains, 
and  the  visible  half  of  the  celestial  sphere 
seems  to  bend  down  on  all  sides  and  to  rest 


io  The  Celestial  Sphere 

upon  the  earth  in  a  circle  which  extends  all 
around  us.  This  circle,  where  the  heavens 
and  the  earth  appear  to  meet,  is  called  the 
horizon.  As  we  ordinarily  see  it,  the  horizon 
appears  irregular  and  broken  on  account  of 
the  unevenness  of  the  earth's  surface,  but  if 
we  are  at  sea,  or  in  the  midst  of  a  great  level 
prairie,  the  horizon  appears  as  a  smooth 
circle,  everywhere  equally  distant  from  the 
eye.  This  circle  is  called  the  sensible  horizon. 
But  there  is  another,  ideal,  horizon,  used  in 
astronomy,  which  is  called  the  rational 
horizon.  It  is  of  the  utmost  importance 
that  we  should  clearly  understand  what  is 
meant  by  the  rational  horizon,  and  for  this 
purpose  we  must  consider  another  fact  con- 
cerning the  dome  of  the  sky. 

We  now  turn  our  attention  to, the  centre 
of  that  dome,  which,  of  course,  is  the  point 
directly  overhead.  This  point,  which  is 
of  primary  importance,  is  called  the  zenith. 
The  position  of  the  zenith  is  indicated  by 
the  direction  of  a  plumb-line.  If  we  imagine 
a  plumb-line  to  be  suspended  from  the 
centre  of  the  sky  overhead,  and  to  pass 
into  the  earth  at  our  feet,  it  would  run  through 
the  centre  of  the  earth,  and,  if  it  were  con- 
tinued onward  in  the  same  direction,  it 


Horizon,  ZenitH,  and  Meridian     II 

would,  after  emerging  from  the  other  side  of 
the  earth,  reach  the  centre  of  the  invisible 
half  of  the  sky-dome  at  a  point  diametrically 
opposite  to  the  zenith.  This  central  point 
of  the  invisible  half  of  the  celestial  sphere, 
lying  under  our  feet,  is  called  the  rAadir. 

Keeping  in  mind  the  definitions  of  zenith 
and  nadir  that  have  just  been  given,  we  are 
in  a  position  to  understand  what  the  rational 
horizon  is.  It  is  a  great  circle  whose  plane 
cuts  through  the  centre  of  the  earth,  and 
which  is  situated  exactly  half-way  between 
the  zenith  and  the  nadir.  This  plane  is 
necessarily  perpendicular,  or  at  right  angles, 
to  the  plumb-line  joining  the  zenith  and  the 
nadir.  In  other  words,  the  rational  horizon 
divides  the  celestial  sphere  into  two  precisely 
equal  halves,  an  upper  and  a  lower  half. 
In  a  hilly  or  mountainous  country  the  sensible 
or  visible  horizon  differs  widely  from  the 
rational,  or  true  horizon,  but  at  sea  the  two 
are  nearly  identical.  This  arises  from  the 
fact,  that  the  earth  is  so  excessively  small  in 
comparison  with  the  distances  of  most  of  the 
heavenly  bodies  that  it  may  be  regarded  as 
a  mere  point  in  the  midst  of  the  celestial 
sphere. 

Besides  the  horizon  and  the  zenith  there 


12  THe  Celestial  SpKere 

is  one  other  thing  of  fundamental  importance 
which  we  must  learn  about  before  proceed- 
ing further, — the  meridian.  The  meridian 


Fig,  i.     The  Rational  and  the  Sensible  Horizon. 

Let  C  be  the  earth's  centre,  O  the  place  of  the  observer, 
and  H  D  the  rational  horizon  passing  through  the  centre 
of  the  earth.  For  an  object  situated  near  the  earth,  as 
at  A,  the  sensible  horizon  makes  a  large  angle  with  the 
rational  horizon.  If  the  object  is  farther  away,  as  at  B, 
the  angle  becomes  less;  and  still  less,  again,  if  the  object 
is  at  D.  It  is  evident  that  if  the  object  be  immensely 
distant,  like  a  star,  the  sensible  horizon  O  S  will  be 
practically  parallel  with  the  rational  horizon,  and  will 
blend  with  it,  because  the  radius,  or  semi-diameter,  of 
the  earth,  O  C,  is  virtually  nothing  in  comparison  with 
the  distance  of  the  star. 

is  an  imaginary  line,  or  semicircle,  beginning 
at  the  north  point  on  the  horizon,  running 
up  through  the  zenith,  and  then  curving 
down  to  the  south  point.  It  thus  divides  the 
visible  sky  into  two  exactly  equal  halves, 
an  eastern  and  a  western  half.  In  the 
ordinary  affairs  of  life  we  usually  think  only 
of  that  part  of  the  meridian  which  extends 
from  the  zenith  to  the  south  point  on  the 
horizon  (which  is  sometimes  called  the 
" noon-line"  because  the  sun  crosses  it  at 
noon),  but  in  astronomy  the  northern  half 


Altitude  and  AzimvitH  13 

of   the   meridian   is   as    important    as    the 
southern. 

4.  Altitude  and  Azimuth.  Now,  suppose 
that  we  wish  to  indicate  the  location  of  a 
star,  or  other  object,  in  the  sky.  To  do  so, 
we  must  have  some  fixe^i  basis  of  reference, 
and  such  a  basis  is  furnished  by  the  horizon 
and  the  zenith.  If  we  tried  to  describe  the 
position  of  a  star,  the  most  natural  thing 
would  be,  first,  to  estimate,  or  measure, 
its  height  above  the  horizon,  and,  second,  to 
indicate  the  direction  in  which  it  was  situated 
with  regard  to  the  points  of  the  compass. 
These  two  measures,  if  they  were  accurately 
made,  would  enable  another  person  to  find 
the  star  in  the  sky.  And  this  is  precisely 
what  is  done  in  astronomy.  The  height 
above  the  horizon  is  called  altitude,  and  the 
bearing  with  reference  to  the  points  of  the 
compass  is  called  azimuth.  Together  these 
are  known  as  co-ordinates.  In  order  to 
systematise  this  method  of  measuring  the 
location  of  a  star,  the  astronomer  uses  ima- 
ginary circles  drawn  on  the  celestial  sphere. 
The  horizon  and  the  meridian  are  two  of 
these  circles.  In  addition  to  these,  other 
imaginary  circles  are  drawn  parallel  to  the 
horizon  and  becoming  smaller  and  smaller 


14  THe  Celestial  SpHere 

until  the  uppermost  one  may  run  close  round 
the  zenith,  which  is  the  common  centre  of 


uth. 


C  is  the  place  of  the  observer. 

N  C  S,  a  north-and-south  line  drawn  in  the  plane  of  the 

horizon. 

E  C  W,  an  east-and-west  line  in  the  plane  of  the  horizon. 

.  N  E  S  W,  the  circle  of  the  horizon. 

Z,  the  observer's  zenith. 

N  Z  S,  vertically  above  N  C  S,  the  meridian. 
•  E  Z  W,  the  prime  vertical. 

Z  s  s',  part  of  a  vertical  circle  drawn  through  the  star  s. 

The  circle  through  s  parallel  to  the  horizon  is  an  altitude 

circle. 
'  The  angle  s  C  s',  or  the  arc  s'  s,  represents  the  star's 

altitude. 

The  angle  s  C  Z,  or  the  arc  Z  s,  is  the  star's  zenith 

distance. 

To  find  the  azimuth,  the  angular  distance  round  the 

horizon  from  S  (o°),  through  W,  N,  E,  to   the   point 

where  the  star's  vertical  circle  meets  the  horizon,   is 

measured.     In  this  case  it  is  315°.     But  if  we  measured 

it  eastward  from  the  south  point  it  would  be  —  45°. 

the    entire    set.     These    are  called  altitude 
circles,    because    each    one    throughout    its 


Altitude  and  AzimxitH  15 

whole  extent  is  at  an  unvarying  height,  or 
altitude,  above  the  horizon.  Such  circles 
may  be  drawn  anywhere  we  please,  so  as  to 
pass  through  any  chosen  star  or  stars.  If 
two  stars  in  different  quarters  of  the  sky 
are  found  to  lie  on  the  same  circle,  then  we 
know  that  both  have  the  same  altitude. 

Then  another  set  of  circles  is  drawn  per- 
pendicular to  the  horizon,  and /SI  intersecting 
at  the  zenith  and  the  nadir.  These  are 
called  vertical  circles,  from  the  fact  that  they 
are  upright  to  the  horizon.  That  one  of 
the  vertical  circles  which  cuts  the  horizon 
at  the  north  and  south  points  coincides 
with  the  meridian,  which  we  have  already 
described .  The  vertical  pjjrcle  at  right 
angles  to  the  meridian  is  called  the  prime 
vertical.  It  cuts  the  horizon  at  the  east  and 
west  points,  dividing  the  visible  sky  into  a 
northern  and  a  southern  half.  Like  the 
altitude  circles,  vertical  circles  may  be  drawn 
anywhere  we  please  so  as  to  pass  through  a 
star  in  any  quarter  of  the  sky — but  the 
meridian  and  the  prime  vertical  are  fixed. 

With  the  two  sets  of  circles  that  have 
just  been  described,  it  is  possible  to  indicate 
accurately  the  location  of  any  heavenly 
body,  at  any  particular  moment.  Its  al- 


1 6  THe  Celestial  SpHere 

titude  is  ascertained  by  measuring,  along  the 
vertical  circle  passing  through  it,  its  distance 
from  the  horizon.  (Sometimes  it  is  con- 
venient to  measure,  instead  of  the  altitude 
of  a  star,  its  zenith  distance,  which  is  also 
reckoned  on  the  vertical  circle.) 

To  ascertain  the  azimuth,  we  must  first 
choose  a  -point  of  beginning  on  the  horizon. 
Any  of  the  cardinal  points,  i.e.,  east,  west, 
north,  or  south,  may  be  employed  for  this 
purpose,  but  in  astronomy  it  is  customary 
to  use  only  the  south  point,  and  to  carry  the 
measure  westward  all  round  the  circle  of  the 
horizon,  and  so  back  to  the  point  of  begin- 
ning in  the  south.  This  involves  circular, 
or  degree,  measure,  to  which  a  few  words 
must  now  be  devoted. 

Every  circle,  no  matter  how  large  or  how 
small,  is  divided  into  360  equal  parts,  called  de- 
grees, usually  indicated  by  the  sign  (°) ;  each 
degree  is  subdivided  into  60  equal  parts  called 
minutes,  indicated  by  the  sign  (') ;  and  each 
minute  is  subdivided  into  60  equal  parts 
called  seconds,  indicated  by  the  sign  ("). 
Thus  there  are  360°,  or  21,600',  or  1,296,000'' 
in  every  complete  circle.  The  actual  length 
of  a  degree  in  inches,  yards,  or  miles,  depends 
upon  the  size  of  the  circle,  but  no  circle  ever 


Altitude  and  AzimvitH  17 

has  more  than  360°,  and  a  degree  of  any 
particular  circle  is  precisely  equal  to  any 
other  degree  of  that  same  circle.  Thus,  if 
a  circle  is  360  miles  in  circumference,  every 
one  of  its  degrees  will  be  one  mile  long. 
In  mathematics,  a  degree  usually  means 
not  a  distance  measured  along  the  circum- 
ference of  a  circle,  but  an  angle  formed  at 
the  centre  of  the  circle  between  two  lines 
called  radii  (radius  in  the  singular),  which 
lines,  where  they  intersect  the  circumference, 
are  separated  by  a  distance  equal  to  one 
36oth  of  the  entire  circle.  But,  for  ordinary 
purposes,  it  is  simpler  to  think  of  a  degree 
as  an  arc  equal  in  length  to  one  36oth  of 
the  circle.  Now,  since  the  horizon,  and 
the  other  imaginary  lines  drawn  in  the  sky, 
are  all  circles,  it  is  evident  that  the  principle 
of  circular  measure  may  be  applied  to  them, 
and  indeed  must  be  so  applied  in  order  that 
they  shall  be  of  use  to  us  in  indicating  the 
position  of  a  star. 

To  return,  then,  to  the  measurement  of 
the  azimuth  of  a  star.  Since  the  south 
point  is  the  place  of  beginning,  we  mark  it 
o°,  and  we  divide  the  circle  of  the  horizon 
into  360°,  counting  round  westward.  Sup- 
pose we  see  a  star  somewhere  in  the  south- 


1 8  The  Celestial  Sphere 

western  quarter  of  the  sky;  then  the  point 
where  the  vertical  circle  passing  through 
that  star  intersects  the  horizon  will  indicate 
its  azimuth.  Suppose  that  this  point  is 
found  to  be  25°  west  of  south;  then  25°  will 
be  the  star's^azimuth.  Suppose  it  is  90°; 
then  the  azimuth  is  90°,  and  the  star  must 
be  on  the  prime  vertical  in  the  west,  because 
west,  being  one  quarter  of  the  way  round  the 
horizon  from  south,  is  90°  in  angular  distance 
from  the  south  point.  Suppose  the  azimuth 
is  1 80° ;  then  the  star  must  be  on  the  meridian 
north  of  the  zenith,  because  north  is  exactly 
half-way,  or  180°  round  the  horizon  from  the 
south  point.  Suppose  the  azimuth  is  270°; 
then  the  star  must  be  on  the  prime  vertical 
in  the  east,  because  east  is  270°,  or  three 
quarters  of  the  way  round  from  the  south 
point.  If  the  star  is  on  the  meridian  in 
the  south  its  azimuth  may  be  called  either 
o°  or  360°,  because  on  any  graduated  circle 
the  mark  indicating  360°  coincides  in  position 
with  o°,  that  being  at  the  same  time  the 
point  of  beginning  and  the  point  of  ending. 
The  same  system  of  angular  measure  is 
applied  in  ascertaining  a  star's  altitude. 
Since  the  horizon  is  half-way  between  the 
zenith  and  the  nadir  it  must  be  just  90° 


Apparent  Motion  of  tHe  Heavens    19 

from  either.  If  a  star  is  in  the  zenith,  then 
its  altitude  is  90°,  and  if  it  is  below  the  zenith 
its  altitude  lies  somewhere  between  o°  and 
90°.  In  any  case  it  cannot  be  less  than  o° 
nor  more  than  90°.  Having  measured  the 
altitude  and  the  azimuth  we  have  the  two 
co-ordinates  which  are  needed  to  indicate 
accurately  the  place  of  a  star  in  the  sky. 
But,  as  we  shall  see  in  a  •moment,  other 
co-ordinates  beside  altitude  and  azimuth 
are  needed  for  a  complete  description  of  the 
places  of  the  stars  on  the  celestial  sphere. 
Owing  to  the  apparent  revolution  of  the 
heavens  round  the  earth,  the  altitudes  and 
azimuths  of  the  celestial  bodies  are  continu- 
ally changing.  We  shall  now  study  the  causes 
of  these  changes. 

5.  The  Apparent  Motion  of  the  Heavens. 
Wo  have  likened  the  earth  to  a  rotating 
school  globe.  As  such  a  globe  turns,  any 
particular  spot  on  it  is  presented  in  succession 
toward  the  various  sides  of  the  room.  In 
precisely  the  same  way  any  spot  on  the  earth 
is  turned  by  its  rotation  successively  toward 
various  parts  of  the  surrounding  sky.  To 
understand  the  effect  of  this,  a  little  patient 
watching  of  the  actual  heavens  will  be 
required,  but  this  has  the  charm  of  all  out- 


2O  TKe  Celestial  SpHere 

of -doors  observation  of  nature,  and  it  will 
be  found  of  fascinating  interest  as  the  facts 
begin  to  unfold  themselves. 

It  is  best  to  begin  by  finding  the  North 
Star,  or  pole  star.  If  you  are  living  not  far 
from  latitude  40°  north,  which  is  the  median 
latitude  of  the  United  States,  you  must, 
after  determining  as  closely  as  you  can  the 
situation  of  the  north  point,  look  upward 
along  the  meridian  in  the  north  until  your 
eyes  are  directed  to  a  point  about  40°  above 
the  horizon.  Forty  degrees  is  somewhat 
less  than  half-way  from  the  horizon  to  the 
zenith,  which,  as  we  have  seen,  are  separated 
by  an  arc  of  90°.  At  that  point  you  will 
notice  a  lone  star  of  what  astronomers  call 
the  second  magnitude.  This  is  the  cele- 
brated North  Star.  It  is  the  most  useful  to 
man  of  all  the  stars,  except  the  sun,  and  it 
differs  from  all  the  others  in  a  way  presently 
to  be  explained.  But  first  it  is  essential 
that  you  should  make  no  mistake  in  identi- 
fying it.  There  are  certain  landmarks  in 
the  sky  which  make  such  identification 
certain.  In  the  first  place,  it  is  always  so 
close  to  the  meridian  in  the  north,  that  by 
naked-eye  observation  you  would  probably 
never  suspect  that  it  was  not  exactly  on  the 


The  Moon  Near  the  "Crater"  Tycho 

Photographed  at  the  Lick  Observatory  under  the  direction  of  E.  S.  Holden. 
Tycho  is  the  regular  oval  depression  a  little  below  the  centre  of  the  view. 
The  vast  depression,  140  miles  across,  with  a  row  of  smaller  craters  within, 
below  the  centre  of  the  view  at  the  top,  is  Clavius.  The  photograph  was 
made  when  sundown  was  approaching  on  that  part  of  the  moon.  Observe 
the  jagged  line  of  adyancing  night  lying  across  the  rugged  surface  on  the 
western  (left-hand)  side. 


Apparent  Motion  of  tKe  Heavens    21 

meridian.  Then,  its  altitude  is  always  equal, 
or  very  nearly  equal,  to  the  latitude  of  the 
place  where  you  happen  to  be  on  the  earth, 
so  that  if  you  know  your  latitude  you  know 
how  high  to  carry  your  eye  above  the  north- 
ern hbrizon.  If  you  are  in  latitude  50°, 
the  star  will  be  at  50°  altitude,  and  if  your 
latitude  is  30°,  the  altitude  of  the  star  will 
be  30°.  Next  you  will  notice  that  the  North 
Star  is  situated  at  the  end  of  the  handle 
of  a  kind  of  dipper-shaped  figure  formed  by 
stars,  the  handle  being  bent  the  wrong  way. 
All  of  the  stars  forming  this  ''dipper"  are 
faint,  except  the  two  which  are  farthest  from 
the  North  Star,  in  the  outer  edge  of  the  bowl, 
one  of  which  is  about  as  bright  as  the  North 
Star  itself.  Again,  if  you  carry  your  eye 
along  the  handle  to  the  bowl,  and  then 
continue  onward  about  as  much  farther, 
you  will  be  led  to  another,  larger,  more 
conspicuous,  and  more  perfect,  dipper- 
shaped  figure,  which  is  in  the  famous  con- 
stellation of  Ursa  Major,  or  the  Great  Bear. 
This  striking  figure  is  called  the  Great 
Dipper  (known  in  England  as  The  Wain). 
It  contains  seven  conspicuous  stars,  all 
of  which,  with  one  exception,  are  equal  in 
brightness  to  the  North  Star.  Now,  look 


22  THe  Celestial  SpKere 

particularly  at  the  two  stars  which  indicate 
the  outer  side  of  the  bowl  of  this  dipper, 
and  you  will  find  that  if  you  draw  an  imagin- 
ary line  through  them  toward  the  meridian 
in  the  north,  it  will  lead  your  eye  directly 
back  to  the  North  Star.  These  two  sig- 
nificant stars  are  often  called  The  Pointers. 
With  their  aid  you  can  make  sure  that  you 
have  really  found  the  North  Star. 

Having  found  it,  begin  by  noting  the 
various  groups  of  stars,  or  constellations, 
in  the  northern  part  of  the  sky,  and,  as  the 
night  wears  on,  observe  whether  any  change 
takes  place  in  their  position.  To  make  our 
description  more  definite,  we  will  suppose 
that  the  observations  begin  at  nine  o'clock 
P.M.  about  the  1st  of  July.  At  that  hour 
and  date,  and  from  the  middle  latitudes  of 
the  United  States,  the  Great  Dipper  is  seen 
in  a  south-westerly  direction  from  the  North 
Star,  with  its  handle  pointing  overhead. 
At  the  same  time,  on  the  opposite  side  of  the 
North  Star,  and  low  in  the  north-east, 
appears  the  remarkable  constellation  of 
Cassiopeia,  easily  recognisable  by  a  zigzag 
figure,  roughly  resembling  the  letter  W, 
formed  by  its  five  principal  stars.  Fix  the 
relative  positions  of  these  constellations  in 


Apparent  Motion  of  tHe  Heavens    23 

the  memory,  and  an  hour  later,  at  10  P.M., 
look  at  them  again.  You  will  find  that  they 
have  moved,  the  Great  Dipper  sinking,  while 
Cassiopeia  rises.  Make  a  third  observation 
at  ii  P.M.,  and  you  will  perceive  that  the 
motion,  has  continued,  The  Pointers  having 
descended  in  the  north-west,  until  they  are 
on  a  level  with  the  North  Star,  while  Cas- 
siopeia has  risen  to  nearly  the  same  level 
in  the  north-east.  In  the  meantime,  the 
North  Star  has  remained  apparently  motion- 
less in  its  original  position.  If  you  repeat 
the  observation  at  midnight,  you  will 
find  that  the  Great  Dipper  has  descended 
so  far  that  its  centre  is  on  a  level  with 
the  North  Star,  and  that  Cassiopeia  has 
proportionally  risen  in  the  north-east.  It 
is  just  as  if  the  two  constellations  were 
attached  to  the  ends  of  a  rod  pivoted  at  the 
centre  upon  the  North  Star  and  twirling 
about  it. 

In  the  meantime  you  will  have  noticed 
that  the  figure  of  the  "Little  IJifi-^tEaS" 
attached  to  the  North  Star,  vjJat  they  go 
bowl  toward  the  zenith  at  9  P. .ere,  or  rather 
round  so  that  at  midnight  ^  and  that  at 
toward  the  south-west.  /s  they  return  to 
perceive  that  the  North  Jut  you  can  do  this 


24  XHe  Celestial  SpHere 

round  which  the  heavens  appear  to  turn, 
carrying  the  other  stars  with  them. 

To  convince  yourself  that  this  motion  is 
common  to  the  stars  in  all  parts  of  the  sky, 
you  should  also  watch  the  conduct  of  those 
which  pass  overhead,  and  those  which  are 
in  the  southern  quarter  of  the  heavens.  For 
instance,  at  9  P.M.  (same  date),  you  will 
see  near  the  zenith  a  beautiful  coronet 
which  makes  a  striking  appearance  although 
all  but  one  of  its  stars  are  relatively  faint. 
This  is  the  constellation  Corona  Borealis, 
or  the  Northern  Crown.  As  the  hours  pass 
you  will  see  the  Crown  swing  slowly  west- 
ward, descending  gradually  toward  the 
horizon,  and  if  you  persevere  in  your  ob- 
servations until  about  5  A.M.,  you  will  ^see 
it  set  in  the  north-west.  The  curve  that  it 
describes  is  concentric  with  those  followed 
by  the  Great  Dipper  and  Cassiopeia,  but, 
being  farther  from  the  North  Star  than  they 
are,  and  at  a  distance  greater  than  the 
Nortrieo9^  tne  North  Star,  it  sinks  below 
appears  the^e^ore  ^  can  arrive  at  a  point 
Cassiopeia  e£rneatn  that  star.  Then  take 
figure,  roughly16  south.  At  9  o'clock,  you 
formed  by  its  frvbrignt  reddish  star  Antares, 
relative  positions  \n  Scorpio,  rather  low  in 


Apparent  Motion  of  tKe  Heavens  25 

the  south  and  considerably  east  of  the 
meridian.  Hour  after  hour  it  will  move 
westward,  in  a  curve  larger  than  that  of  the 
Northern  Crown,  but  still  concentric  with  it. 
A  little  before  10  P.M.,  it  will  cross  the 
meridian,  and  between  I  and  2  A.M.,  it 
will  sink  beneath  the  horizon  at  a  point  south 
of  west. 

So,  no  matter  in  what  part  of  the  heavens 
you  watch  the  stars,  you  will  see  not  only 
that  they  move  from  east  to  west,  but  that 
this  motion  is  performed  in  curves  concentric 
round  the  North  Star,  which  alone  appears 
to  maintain  its  place  unchanged.  Along 
the  eastern  horizon  you  will  perceive  stars 
continually  rising;  in  the  middle  of  the  sky 
you  will  see  others  continually  crossing  the 
meridian — a  majestic  march  of  constellations, 
— and  along  the  western  horizon  you  will  find 
still  others  continually  setting.  If  you  could 
watch  the  stars  uninterruptedly  through- 
out the  twenty-four  hours  (if  daylight  did 
not  hide  them  from  sight  during  half  that 
period),  -you  would  perceive  that  they  go 
entirely  round  the  celestial  sphere,  or  rather 
that  it  goes  round  with  them,  and  that  at 
the  end  of  twenty-four  hours  they  return  to 
their  original  positions.  But  you  can  do  this 


26  THe  Celestial  SpKere 

just  as  well  by  looking  at  them  on  two  suc- 
cessive nights,  when  you  will  find  that  at  the 
same  hour  on  the  second  night  they  are  back 
again,  practically  in  the  places  where  you 
saw  them  on  the  first  night. 

Of  course,  what  happens  on  a  July  night 
happens  on  any  other  night  of  the  year. 
We  have  taken  a  particular  date  merely 
in  order  to  make  the  description  clearer. 
It  is  only  necessary  to  find  the  North  Star, 
the  Great  Dipper,  and  Cassiopeia,  and  you 
can  observe  the  apparent  revolution  of  the 
heavens  at  any  time  of  the  year.  These 
constellations,  being  so  near  the  North 
Star  that  they  never  go  entirely  below  the 
horizon  in  middle  northern  latitudes,  are 
always  visible  on  one  side  or  another  of  the 
North  Star. 

Now,  call  upon  your  imagination  to  deal 
with  what  you  have  been  observing,  and  you 
will  have  no  difficulty  in  explaining  what 
all  this  apparent  motion  of  the  stars  means. 
You  already  know  that  the  heavens  form 
a  sphere  surrounding  the  earth.  You  have 
simply  to  suppose  the  North  Star  to  be 
situated  at,  or  close  to,  the  north  end,  or 
north  pole,  of  an  imaginary  axle,  or  axis, 
round  which  the  celestial  sphere  seems  to 


Apparent  Motion  of  tHe  Heavens  27 

turn,  and  instantly  the  whole  series  of 
phenomena  will  fall  into  order,  and  the 
explanation  will  stare  you  in  the  face.  That 
explanation  is  that  the  motion  of  all  the 
stars  in  concentric  circles  round  the  North 
Star  is  due  to  an  apparent  revolution  of 
the  whole  celestial  sphere,  like  a  huge 
hollow  ball,  about  an  axis,  the  position  of 
one  of  whose  poles  is  graphically  indicated 
in  the  sky  by  the  North  Star.  The  cir- 
cles in  which  the  stars  seem  to  move  are 
perpendicular  to  this  axis,  and  inclined 
to  the  horizon  at  an  angle  depending 
upon  the  altitude  of  the  North  Star  at  the 
place  on  the  earth  where  the  observations 
are  made. 

Another  important  fact  demands  our 
attention,  although  the  thoughtful  reader 
will  already  have  guessed  it — the  north  pole 
of  the  celestial  sphere,  whose  position  in  the 
sky  is  closely  indicated  by  the  North  Star, 
is  situated  directly  over  the  north  pole 
of  the  earth.  This  follows  from  the  fact  that 
the  apparent  revolution  of  the  celestial 
sphere  is  due  to  the  real  rotation  of  the  earth. 
You  can  see  that  the  two  poles,  that  of  the 
earth  and  that  of  the  heavens,  must  neces- 
sarily coincide,  by  taking  a  school  globe  and 


28  The  Celestial  Sphere 

imagining  that  you  are  an  intelligent  little 
being  dwelling  on  its  surface.  As  the  globe 
turned  on  its  axis  you  would  see  the  walls  of 
the  room  revolving  round  you,  and  the  poles 
of  the  apparent  axis  round  which  the  room 
turned,  would,  evidently,  be  directly  over 
the  corresponding  poles  of  the  globe  itself. 
Another  thing  which  you  could  make  clear 
by  this  experiment  is  that,  as  the  poles  of 
the  celestial  sphere  are  over  the  earth's 
poles,  so  the  celestial  equator,  or  equator 
of  the  heavens,  must  be  directly  over  the 
equator  of  the  earth. 

We  can  determine  the  location  of  the 
poles  of  the  heavens  by  watching  the  revo- 
lution of  the  stars  around  them,  and  we  can 
fix  the  position  of  the.  circle  of  the  equator 
of  the  heavens,  by  drawing  an  imaginary 
line  round  the  celestial  sphere,  half-way 
between  the  two  poles.  We  have  spoken 
specifically  only  of  the  north  pole,  but,  of 
course,  there  is  a  corresponding  south  pole 
situated  over  the  south  pole  of  the  earth, 
but  whose  position  is  invisible  from  the 
northern  hemisphere.  It  happens  that  the* 
place  of  the  south  celestial  pole  is  not  in- 
dicated to  the  eye,  like  that  of  the  northern, 
by  a  conspicuous  star. 


Drawing  of  Jupiter 


Drawing  of  Jupiter 

Note  the  change  of  details  between  the  two  drawings,  made  at  different 
times.     Similar  changes  are  continually  occurring. 


Locating  tHe  Stars  29 

6.  Locating  the  Stars  on  the  Celestial 
Sphere.  Having  found  the  poles  and  the 
equator  of  the  celestial  sphere,  we  begin 
to  see  'how  it  is  possible  to  make  a  map, 
or  globe,  of  the  heavens  just  as  we  do  of  the 
earth,  on  which  the  objects  that  they  contain 
may  be  represented  in  their  proper  positions. 
When  we  wish  to  describe  the  location  of 
an  object  on  the  earth,  a  city  for  instance, 
we  have  to  refer  to  a  system  of  imaginary 
circles,  drawn  round  the  earth  and  based 
upon  theJfcuator  and  the  poles.  These 
circles  enalS  us  to  fix  the  place  of  any  point 
on  the  earth  with  accuracy.  One  set  of 
circles  called  parallels  of  latitude  are  drawn 
east  and  west  round  the  globe  parallel  to 
the  equator,  and  becoming  smaller  and 
smaller  until  the  smallest  runs  close  round 
their  common  central  point,  which  is  one 
of  the  poles.  Each  pole  of  the  earth  is  the 
centre  of  such  a  set  of  circles  all  parallel  to 
the  equator.  Since  each  circle  is  unvarying 
in  its  distance  from  the  equator,  all  places 
which  are  situated  anywhere  on  that  circle 
have  the  same  latitude,  or  distance  from  the 
equator,  either  north  or  south. 

But  to  know  the  latitude  of  any  place  on 
the  earth  is  not  sufficient;  we  must  also 


30  "THe  Celestial  SpHere 

know  what  is  called  its  longitude,  or  its 
angular  distance  east  or  west  of  some  chosen 
point  on  the  equator.  This  knowledge  is 
obtained  with  the  aid  of  another  set  of  circles 
drawn  north  and  south  round  the  earth, 
and  all  meeting  and  crossing  at  the  poles. 
These  are  called  meridians  of  longitude. 
In  order  to  make  use  of  them  we  must, 
as  already  intimated,  select  some  particular 
meridian  whose  crossing  point  on  the  equator 
will  serve  as  a  place  of  beginning.  By 
common  consent  of  the  civilisa^world,  the 
meridian  which  passes  througn^he  obser- 
vatory at  Greenwich,  near  London,  has  been 
chosen  for  this  purpose.  It  is,  like  all  the 
meridians,  perpendicular  to  the  equator, 
and  it  is  called  the  prime  meridian  of  the 
earth. 

In  locating  any  place  on  the  earth,  then, 
we  ascertain  by  means  of  the  parallel  of 
latitude  passing  through  it  how  far  in 
degrees,  it  is  north  or  south  of  the  equator, 
and  by  means  of  its  meridian  of  longitude 
how  far  it  is  east  or  west  of  the  prime  me- 
ridian, or  meridian  of  Greenwich.  These 
two  things  being  known,  we  have  the  exact 
location  of  the  place  on  the  earth.  Let  us 
now  see  how  a  similar  system  is  applied  to 


Locating  tKe  Stars  31 

ascertain  the  location  of  a  heavenly  body 
on  the  celestial  sphere. 

We  have  observed  that  the  poles  of  the 
heavens  correspond  in  position,  or  direction, 
with  those  of  the  earth,  and  that  the  equator 
of  the  heavens  runs  round  the  sky  directly 
over  the  earth's  equator.  It  follows  that 
we  can  divide  the  celestial  sphere  just  as 
we  do  the  surface  of  the  earth  by  means  of 
parallels  and  meridians,  corresponding  to  the 
similar  circles  of  the  earth.  On  the  earth, 
distance  from  the  equator  is  called  latitude, 
and  distance  from  the  prime  meridian, 
longitude.  In  the  heavens,  distance  from 
the  equator  is  called  declination,  and  distance^ 
from  the  prime  meridian,  right  ascensionj 
but  they  are  essentially  the  same  things 
as  latitude  and  longitude,  and  are  measured 
virtually  in  the  same  way.  In  place  of 
parallels  of  latitude,  we  have  on  the  celestial 
sphere  circles  drawn  parallel  to  the  equator 
and  centring  about  the  celestial  poles, 
which  are  called  parallels  of  declination, 
and  in  place  of  meridians~of  longitude,  we 
have  circles  perpendicular  to  the  equator, 
and  drawn  through  the  celestial  poles, 
which  are  called  hour  circles.  The  origin 
of  this  name  will  be  explained  in  a  moment. 


32  THe  Celestial  SpKere 

For  the  present  it  is  only  necessary  to  fix 
firmly  in  the  mind  the  fact  that  these  two 
systems  of  circles,  one  on  the  earth  and  the 
other  in  the  heavens,  are  fundamentally 
identical. 

Just  as  on  the  earth  geographers  have 
chosen  a  particular  place,  viz.  Greenwich, 
to  fix  the  location  of  the  terrestrial  prime 
meridian,  so  astronomers  have  agreed  upon 
a  particular  point  in  the  heavens  which 
serves  to  determine  the  location  of  the 
celestial  prime  meridian.  This  point,  which 
lies  on  the  celestial  equator,  is  known  as  the 
vernal  equinox.  We  shall  explain  its  origin 
after  having  indicated  its  use.  The  hour 
circle  which  passes  through  the  vernal 
equinox  is  the  prime  meridian  of  the  heavens, 
and  the  vernal  equinox  itself  is  sometimes 
called  the  "  Greenwich  of  the  Sky." 

If,  now,  we  wish  to  ascertain  the  exact 
location  of  a  star  on  the  celestial  sphere,  as 
we  would  that  of  New  York,  London,  or 
Paris,  on  the  earth,  we  measure  along  the  hour 
circle  running  through  it,  its  declination, 
or  distance  from  the  celestial  equator,  and 
then,  along  its  parallel  of  declination,  we 
measure  its  right  ascension,  or  distance 
from  the  vernal  equinox  Having  these 


After  Shifting;  Observer's  Position  33 

two  co-ordinates,  we  possess  all  that  is  nec- 
essary to  enable  us  to  describe  the  position 
of  the  star,  so  that  someone  else  looking  for 
it,  may  find  it  in  the  sky,  as  a  navigator 
finds  some  lone  island  in  the  sea  by  knowing 
its  latitude  and  longitude. 

Decimation,  as  we  have  seen,  is  simply 
another  name  for  latitude,  but  right  ascen- 
sion, which  corresponds  to  longitude,  needs  a 
little  additional  explanation.  It  differs  from 
longitude,  first,  in  that,  instead  of  being  reck- 
oned both  east  and  west  from  the  prime 
meridian,  it  is  reckoned  only  toward  the  east, 
the  reckoning  being  continued  uninterrup- 
tedly entirely  round  the  circle  of  the 
equator;  and,  second,  in  that  it  is  usually 
counted  not  in  degrees,  minutes,  and  seconds 
of  arc,  but  in  hours,  minutes,  and  seconds 
of  time.  The  reason  for  this  is  that, 
since  the  celestial  sphere  makes  one  com- 
plete revolution  in  twenty-four  hours,  it 
is  convenient  to  divide  the  circuit  into 
twenty-four  equal  parts,  each  correspond- 
ing to  the  distance  through  which  the 
heavens  appear  to  turn  in  one  hour.  This 
explains  the  origin  of  the  term  hour  circles 
applied  to  the  celestial  meridians,  which, 
by  intersecting  the  equator,  divide  it  into 


34  XHe  Celestial  SpHere 

twenty-four  equal  parts,  each  part  corre- 
sponding to  an  hour  of  time.  In  ex- 
pressing right  ascension  in  time,  the  Roman 
numerals — i,  n,  in,  iv,  v,  vi,  vn,  vm, 

IX,  X,  XI,  XII,  XIII,  XIV,  XV,  XVI,  XVII, 
XVIII,  XIX,  XX,  XXI,  XXII,  XXIII,  XXIV— 

are  employed  for  the  hours,  and  the  letters 
m  and  s  respectively  for  the  minutes  and 
seconds.  Since  there  are  360°  in  every 
circle,  it  is  plain  that  one  hour  of  right  ascen- 
sion corresponds  to  15°.  So,  too,  one  minute 
of  right  ascension  corresponds  to  15',  and 
one  second  to  15".  It  will  be  found  useful 
to  memorise  these  relations. 

7.  Effects  Produced  by  Changing  the 
Observer's  Place  on  the  Earth.  The  reader 
will  recall  that  in  Sect.  4  we  described  an- 
other system  of  circles  for  determining  the 
places  of  stars,  a  system  based  on  the  horizon 
and  the  zenith.  This  horizon-zenith  system 
takes  no  account  of  the  changes  produced 
by  the  apparent  motion  of  the  heavens, 
and  consequently  it  is  not  applicable  to 
determining  the  absolute  positions  of  the 
stars  on  the  celestial  sphere.  It  simply 
shows  their  positions  in  the  visible  half  of 
the  sky,  as  seen  at  some  particular  time 
from  some  definite  point  on  the  earth.  In 


After  Shifting'  Observer's  Position  35 


Fig.  J.     Right  Ascension  and  Declination. 

The  plane  of  the  horizon,  with  the  north,  south,  east,  and 

west  points,  and  the  zenith,  are  represented  as  in  Fig.  2. 

P  and  P'  are  the  poles  of  the  celestial  sphere,  the  dotted 

line  connecting  them  representing  the  direction  of  the 

axis,  both  of  celestial  sphere  and  the  earth. 

The  circle  Eq  Eq'  is  the  equator. 

V  is  the  vernal  equinox,  or  the  point  on  the  equator 

whence  right  ascension  is  reckoned  round  toward  the 

east. 

The  circle  passing  through  s,  parallel  to  the  equator,  is  a 

declination  circle. 

The  circle  P  s  P'  is  the  hour  circle  of  the  star  s. 

The  arc  of  this  Tiour  circle  contained  between  s  and  the 

point  where  it  meets  the  equator  is  the  star's  declination. 

Its  right  ascension  is  measured  by  the  arc  of  the  equator 

contained  between  V  and  the  point  where  its  hour  circle 

meets  the  equator,  or  by  the  angle  V  P  s. 

The  hour   circle^  P  V  P',    passing   through   the  vernal 

equinox,    is    the    equinoctial    colure.     When    this    has 

moved  up  to  coincidence  with  the  meridian,  N  Z  S,  it 

will  be  astronomical 


36  THe  Celestial  SpHere 

order  to  show  the  changing  relations  of  this 
system  to  that  which  we  have  just  been  de- 
scribing, let  us  consider  the  effects  produced 
by  shifting  our  place  of  observation  on  the 
earth.  Since  the  zenith  is  the  point  overhead 
and  the  nadir  the  point  underfoot,  and  the 
horizon  is  a  great  circle  drawn  exactly  half- 
way between  the  zenith  and  the  nadir,  it 
is  evident,  upon  a  moment's  consideration, 
that  every  place  on  the  earth  has  its  own 
zenith  and  its  own  horizon.  It  is  also  clear 
that  every  place  must  have  its  own  meridian, 
because  the  meridian  is  a  north  and  south 
line  running  directly  over  the  observer's 
head.  You  can  see  how  this  is,  if  you  reflect 
that  for  an  observer  situated  on  the  other 
side  of  the  earth  what  is  overhead  for  you 
will  be  underfoot  for  him,  and  vice  versa. 
Thus  the  direction  of  our  zenith  is  the 
direction  of  the  nadir  for  our  antipodes, 
and  the  direction  of  their  zenith  is  the 
direction  of  our  nadir.  They  see  the  half 
of  the  sky  which  is  invisible  to  us,  and  we 
the  half  which  is  invisible  to  them. 

Now,  suppose  that  we  should  go  to  the 
north  pole.  The  celestial  north  pole  would 
then  be  in  our  zenith,  and  the  equator  would 
correspond  with  the  horizon.  Thus,  for 


After  Shifting  Observer's  Position  37 

an  observer  at  the  north  pole  the  two  systems 
of  circles  that  we  have  described  would  fall 
into  coincidence.  The  zenith  would  cor- 
respond with  the'  pole  of  the  heavens;  the 
horizon  would  correspond  with  the  celestial 
equator ;  the  vertical  circles  would  correspond 
with  the  hour  circles;  and  the  altitude 
circles  would  correspond  with  the  circles  of 
declination.  The  North  Star,  being  close 
to  the  pole  of  the  heavens,  would  appear 
directly  overhead.  Being  at  the  zenith, 
its  altitude  would  be  90°  (see  Sect.  4). 
Peary,  if  he  had  visited  the  pole  during  the 
polar  night,  would  have  seen  the  North 
Star  overhead,  and  it  would  have  enabled 
him  with  relatively  little  trouble  to  deter- 
mine his  exact  place  on  the  earth,  or,  in  other 
words,  the  exact  location  of  the  north 
terrestrial  pole.  With  the  pole  star  in  the 
zenith,  it  is  evident  that  the  other  stars 
would  be  seen  revolving  round  it  in  circles 
parallel  to  the  horizon.  All  the  stars 
situated  north  of  the  celestial  equator  would 
be  simultaneously  and  continuously  visible. 
None  of  them  would  either  rise  or  set,  but 
all,  in  the  course  of  twenty-four  hours,  would 
appear  to  make  a  complete  circuit  horizontally 
round  the  sky.  This  polar  presentation  of  the 


38  The  Celestial  SpKere 

celestial  sphere  is  called  the  parallel  sphere, 
because  the  stars  appear  to  move  parallel 
to  the  horizon.  No  man  has  yet  beheld 
the  nocturnal  phenomena  of  the  parallel 
sphere,  but  if  in  the  future  some  explorer 
should  visit  one  of  the  earth's  poles  during 
the  polar  night,  he  would  behold  the  spectacle 
in  all  its  strange  splendour. 

Next,  suppose  that  you  are  somewhere 
on  the  earth's  equator.  Since  the  equator 
is  everywhere  90°,  or  one  quarter  of  a  circle, 
from  each  pole,  it  is  evident  that  looking  at 
the  sky  from  the  equator  you  would  see  the 
two  poles  (if  there  was  anything  to  mark 
their  places)  lying  on  the  horizon  one  exactly 
in  the  north  and  the  other  exactly  in  the 
south.  The  celestial  equator  would  cor- 
respond with  the  prime  vertical,  passing 
east  and  west  directly  over  your  head,  and 
all  the  stars  would  rise  and  set  perpendicu- 
larly to  the  horizon,  each  describing  a  semi- 
circle in  the  sky  in  the  course  of  twelve 
hours.  During  the  other  twelve  hours,  the 
same  stars  would  be  below  the  horizon.  Stars 
situated  near  either  of  the  poles  would  de- 
scribe little  semi-circles  near  the  north  or 
the  south  point;  those  farther  away  would 
describe  larger  semi-circles;  those  close  to 


Jupiter 

Photographed  at  the  Lick  Observatory. 
Observe  on  the  left  the  Great  Red  Spot,  which  first  appeared  in  1878. 


After  SHifting  Observer's  Position  39 

the  celestial  equator  would  describe  semi- 
circles passing  overhead.  But  all,  no  matter 
where  situated,  would  describe  their  visible 
courses  in  the  same  period  of  time.  This 
equatorial  presentation  of  the  celestial  sphere 
is  called  the  right  sphere,  because  the  stars 
rise  and  set  at  a  right  angle  to  the  plane  of 
the  horizon.  Comparing  it  with  the  system 
of  circles  on  which  right  ascension  and  de- 
clination are  based,  we  see  that,  as  the  prime 
vertical  corresponds  with  the  celestial  equa- 
tor, so  the  horizon  must  represent  an  hour 
circle.  The  meridian  also  represents  an  hour 
circle.  It  may  require  a  little  thought  to 
make  this  clear,  but  it  will  be  a  good 
exercise. 

Finally,  if  you  are  somewhere  between  the 
equator  and  one  of  the  poles,  which  is  the 
actual  situation  of  the  vast  majority  of  man- 
kind, you  see  either  the  north  or  the  south 
pole  of  the  heavens  elevated  to  an  altitude 
corresponding  with  your  latitude,  and  the 
stars  apparently  revolving  round  it  in  circles 
inclined  to  the  horizon  at  an  angle  depending 
upon  the  latitude.  The  nearer  you  are  to 
the  equator,  the  steeper  this  angle  will  be. 
This  ordinary  presentation  of  the  celestial 
sphere  is  called  the  oblique  sphere.  Its 


4-O  XHe  Celestial  SpHere 

horizon  does  not  correspond  with  either  the 
equator  or  the  prime  vertical,  and  its  zenith 
and  nadir  lie  at  points  situated  between  the 
celestial  poles  and  the  celestial  equator. 

8.  The  Astronomical  Clock  and  the  Eclip- 
tic. It  will  be  remembered  that  the  meridian 
of  any  place  on  the  earth  is  a  straight  north 
and  south  line  running  through  the  zenith 
and  perpendicular  to  the  horizon.  More 
strictly  speaking,  the  meridian  is  a  circle 
passing  from  north  to  south  directly  overhead 
and  corresponding  exactly  with  the  meridian 
of  longitude  of  the  place  of  observation. 
Now,  let  us  consider  the  hour  circles  on  the 
celestial  sphere.  They  are  drawn  in  the 
same  way  as  the  meridians  on  the  earth. 
But  the  celestial  sphere  appears  to  revolve 
round  the  earth,  and  as  it  does  so  it  must 
carry  the  hour  circles  with  it,  since  they  are 
fixed  in  position  upon  its  surface.  Fix 
your  attention  upon  the  first  of  these  hour 
circles,  i.  e.,  the  one  which  runs  through  the 
vernal  equinox.  Its  right  ascension  is  called 
o  hours,  because  it  is  the  starting  point. 
Suppose  that  at  some  time  we  find  the  vernal 
equinox  exactly  in  the  south;  then  the  o 
hour  circle,  or  the  prime  meridian  of  the 
heavens,  will,  at  that  instant,  coincide  with 


Astronomical  GlocK  and  Ecliptic    41 

the  meridian  of  the  place  of  observation. 
But  one  hour  later,  in  consequence  of  the 
motion  of  the  heavens,  the  vernal  equinox, 
together  with  the  circle  of  o  hours,  will  be 
15°,  or  one  hour  of  right  ascension,  west 
of  the  meridian,  and  the  hour  circle  marked 
I  will  have  come  up  to,  and  for  an  instant  will 
be  blended  with,  the  meridian.  An  hour 
later  still,  the  circle  of  II  hours  right  ascension 
will  have  taken  its  place  on  the  meridian, 
while  the  vernal  equinox  and  the  circle  of  o 
hours  will  be  n  hours,  or  30°,  west  of  the 
meridian.  And  so  on,  throughout  the  entire 
circuit  of  the  sky. 

What  has  just  been  said  makes  it  evident 
that  the  apparent  motion  of  the  heavens 
resembles  the  movement  of  a  clock,  the 
vernal  equinox,  or  the  circle  of  o  hours, 
serving  as  a  hand,  or  pointer,  on  the  dial. 
Astronomers  use  it  in  exactly  that  way,  for 
astronomical  clocks  are  made  with  dials 
divided  into  twenty-four  hour  spaces,  and  the 
time  reckoning  runs  continuously  from  o 
hours  to  xxiv  hours.  The  "astronomical 
day  "  begins  when  thejvgrnal^eguinox  is  onjhe 
meri^iaji.  At  thaLinstant  the  hands  of  the 
astronomical  clock  mark  o  hours,  o  minutes, 
o  seconds.  Thus  the  clock  follows  the  motion 


42  XKe  Celestial  SpKere 

of  the  heavens,  and  the  astronomer  can  tell 
by  simply  glancing  at  the  dial,  and  without 
looking  at  the  sky,  where  the  vernal  equinox 
is,  and  what  is  the  right  ascension  of  any 
body  which  may  at  that  moment  be  on  the 
meridian. 

We  must  now  explain  a  little  more  fully 
what  the  vernal  equinox  is,  and  why  it  has 
been  chosen  as  the  "Greenwich  of  the  Sky." 
Its  position  is  not  marked  by  any  star,  but 
is  determined  by  means  of  the  intersecting 
circles  that  we  have  described.  There  is 
one  other  such  circle,  that  we  have  not  yet 
mentioned,  which  bears  a  peculiar  relation 
to  the  vernal  equinox.  This  is  the  ecliptic. 
Just  as  the  daily,  or  diurnal,  rotation  of  the 
earth  on  its  axis  causes  the  whole  celestial 
sphere  to  appear  to  make  one  revolution 
every  day,  so  the  yearly  or  annual  revolution 
of  the  earth  in  its  orbit  about  the  sun  causes 
the  sun  to  appear  to  make  one  revolution 
through  the  sky  every  year.  As  the  earth 
moves  onward  in  its  orbit,  the  sun  seems  to 
move  in  the  opposite  direction.  Inasmuch 
as  there  are  360°  in  a  complete  circle  and 
365  days  in  a  year,  the  apparent  motion  of 
the  sun  amounts  to  nearly  i°  per  day,  or 
30°  per  month.  In  twelve  months,  then, 


.Astronorf&ical  ClocK  and  Ecliptic   43 

the  sun  comes  back  again  to  the  place  in  the 
sky  which  it  occupied  at  the  beginning  of  the 
year.  Since  the  motion  of  the  earth  in  its 
orbit  is  from  west  to  east  (the  same  as  that 
of  its  rotation  on  its  axis),  it  follows  that 
the  direction  of  the  sun's  apparent  annual 
motion  in  the  sky  is  from  east  to  west 
(like  its  daily  motion).  Thus,  while  in 
Jact  the  earth  pursues  a  path,  or  orbit,  round 
the  sun,  the  sun  seems  to  pursue  a  path 
round  the  earth.  This  apparent  path  of 
the  sun,  projected  against  the  background 
of  the  sky,  is  called  the  ecliptic.  The  name 
arises  from  the  fact  that  eclipses  only  occur 
when  the  moon  is  in  or  near  the  plane  of  the 
sun's  apparent  path. 

As  the  apparent  motion  of  the  sun  round 
the  ecliptic  is  caused  by  the  real  motion 
of  the  earth  round  the  sun,  we  may  regard 
the  ecliptic  as  a  circle  marking  the  inter- 
section of  the  plane  of  the  earth's  orbit 
with  the  celestial  sphere.  In  other  words, 
if  we  were  situated  on  the  sun  instead  of  on 
the  earth,  we  would  see  the  earth  travelling 
round  the  sky  in  the  circle  of  the  ecliptic. 
We  must  keep  this  fact,  that  the  ecliptic  in- 
dicates the  plane  of  the  earth's  orbit,  firmly 
in  mind,  in  order  to  understand  what  follows. 


44  THe  Celestial  SpHere 

The  ecliptic  is  not  coincident  with  the 
celestial  equator,  for  the  following  reason: 
The  axis  of  the  earth's  daily  rotation  is  not 
parallel  to,  or  does  not  point  in  the  same 
direction  as,  the  axis  of  its  yearly  revolution 
round  the  sun.  As  the  axis  of  rotation  is 
perpendicular  to  the  equator,  so  the  axis  of 
the  yearly  revolution  is  perpendicular  to 
the  ecliptic,  and  since  these  two  axes  are 
inclined  to  one  another,  it  results  that  the 
equator  and  the  ecliptic  must  lie  in  different 
planes.  The  inclination  of  the  plane  of  the 
ecliptic  to  that  of  the  equator  amounts  to 
about  23^°. 

As  it  is  very  important  to  have  a  clear 
conception  of  this  subject,  we  may  illustrate 
it  in  this  way:  Take  a  ball  to  represent  the 
earth,  and  around  it  draw  a  circle  to  represent 
the  equator.  Then,  through  the  centre  of 
the  ball,  and  at  right  angles  to  its  equator, 
put  a  long  pin  to  represent  the  axis.  Set  it 
afloat  in  a  tub  of  water,  weighting  it  so  that 
it  will  be  half  submerged,  and  placing  it  in 
such  a  position  that  the  pin  will  be  not  upright 
but  inclined  at  a  considerable  angle  from  the 
vertical.  Now,  imagine  that  the  sun  is 
situated  in  the  centre  of  the  tub,  and  cause 
the  ball  to  circle  slowly  round  it,  while 


Astronomical  ClocK  and  Ecliptic    45 

maintaining  the  pin  always  in  the  same  po- 
sition. Then  the  surface  of  the  water  will 
represent  the  plane  of  the  ecliptic,  or  plane 
of  the  earth's  orbit,  and  you  will  see  that, 
in  consequence  of  the  inclination  of  the  pin, 
the  plane  of  the  equator  does  not  coincide 
with  that  of  the  ecliptic  (or  the  surface  of 
the  water),  but  is  tipped  with  regard  to  it 
in  such  a  manner  that  one  half  of  the  equator 
is  below  and  the  other  half  above/ it.  Instead 
of  actually  trying  this  experiment,  it  will  be 
a  useful  exercise  of  the  imagination  to  re- 
present it  to  the  mind's  eye  just  as  if  it 
were  tried. 

We  have  said  that  the  inclination  of  the 
equator  to  the  ecliptic  amounts  to  .23^2°, 
and  this  angle  should  be  memorised.  Now, 
since  both  the  ecliptic  and  the  equator  are 
great  circles  of  the  celestial  sphere,  L  e., 
circles  whose  planes  cut  through  the  centre 
of  the  sphere,  they  must  intersect  one 
another  at  two  opposite  points.  In  the 
experiment  just  described,  these  two  points 
lie  on  opposite  sides  of  the  ball,  where  the 
equator  cuts  the  level  of  the  water.  These 
points  of  intersection  of  the  equator  and  the 
ecliptic  on  the  celestial  sphere  are  called 
the  equinoxes,  or  equinoctial  points,  because 


46  XHe  Celestial  SpHere 

when  the  sun  appears  at  either  of  those  points 
it  is  perpendicular  over  the  equator,  and  when 
it  is  in  that  position  day  and  night  are  of 
equal  length  all  over  the  earth.  (Equinox 
is  from  two  Latin  words  meaning  "equal 
night/') 

We  shall  have  more  to  say  about  the 
equinoxes  later,  but  for  the  present  it  is 
sufficient  to  remark  that  one  of  these  points 
—that  one  where  the  sun  is  about  the  2ist 
of  March,  which  is  the  beginning  of  astro- 
nomical spring — is  the  "Greenwich  of  the 
Sky,"  or  the  vernal  equinox.  The  other, 
opposite,  point  is  called  the  autumnal 
equinox,  because  the  sun  arrives  there 
about  the  23d  of  September,  the  beginning 
of  astronomical  autumn.  The  vernal  equi- 
nox, as  we  have  already  seen,  serves  as  a 
pointer  on  the  dial  of  the  sky.  When  it 
crosses  the  meridian  of  any  place  it  is 
astronomical  noon  at  that  place.  Its 
position  in  the  sky  is  not  marked  by  any 
particular  star,  but  it  is  situated  in  the 
constellation  Pisces,  and  lies  exactly  at  the 
crossing  point  of  the  celestial  equator  and 
the  ecliptic.  The  hour  circle,  running  through 
this  point,  and  through  its  opposite,  the 
autumnal  equinox,  is  the  prime  meridian  of 


Saturn 

From  a  drawing  by  Trouvelot. 


Saturn 

Photographed  at  the  Lick  Observatory. 


Astronomical  ClocK  and  Ecliptic     47 

the  heavens,  called  the  equinoctial  colure. 
The  hour  circle  at  right  angles  to  the  equi- 
noctial colure,  i.  e.,  bearing  to  it  the  same 
relation  that  the  prime  vertical  does  to 
the  meridian  (see  Sect.  4),  is  called  the  sol- 
stitial colure.  This  latter  circle  cuts 
the  ecliptic  at  two  opposite  points,  called 
the  solstices,  which  lie  half-way  between  the 
equinoxes.  Since  the  ecliptic  is  inclined 
23//2°  to  the  plane  of  the  equator,  and  since 
the  solstices  lie  half-way  between  the  two 
crossing  points  of  the  ecliptic  and  the  equator, 
it  is  evident  that  the  solstices  must  be 
situated  23^/2°  from  the  equator,  one  above 
and  the  other  below,  or  one  north  and  the 
other  south.  The  northern  one  is  called 
the  summer  solstice,  because  the  sun  arrives 
there  at  the  beginning  of  astronomical  sum- 
mer, about  the  22 d  of  June,  and  the  southern 
one  is  called  the  winter  solstice,  because  the 
sun  arrives  there  at  the  beginning  of  the 
astronomical  winter,  about  the  22d  of 
December.  The  name  solstice  comes  from 
two  Latin  words  meaning  "the  standing  still 
of  the  sun,"  because  when  it  is  at  the  solsti- 
tial points  its  apparent  course  through  the 
sky  is  for  several  days  nearly  horizontal 
and  its  declination  changes  very  slowly. 


48  THe  Celestial  SpHere 

Now,  just  as  there  are  two  opposite 
points  in  the  sky  at  equal  distances  from  the 
equator,  which  mark  the  poles  of  the  ima- 
ginary axis  about  which  the  celestial  sphere 
makes  its  diurnal  revolution,  so  there  are 
two  opposite  points  at  equal  distances  from 
the  ecliptic  which  mark  the  poles  of  the 
imaginary  axis  about  which  the  yearly 
revolution  of  the  sun  takes  place.  These 
are  called  the  poles  of  the  ecliptic,  and  they 
are  situated  23^/2°  from  the  celestial  poles — 
a  distance  necessarily  corresponding  with 
the  inclination  of  the  ecliptic  to  the  equator. 
The  northern  pole  of  the  ecliptic  is  in  the 
constellation  Draco,  which  you  may  see  any 
night  circling  round  the  North  Star,  together 
with  the  Great  Dipper  and  Cassiopeia. 

9.  Celestial  Latitude  and  Longitude.  We 
have  seen  that  the  celestial  sphere  is  marked 
with  imaginary  circles  resembling  the  circles 
of  latitude  and  longitude  on  the  earth,  and 
that  in  both  cases  the  circles  are  used  for 
a  similar  purpose,  viz.,  to  determine  the 
location  of  objects,  in  one  case  on  the  globe 
of  the  earth  and  in  the  other  on  the  sphere 
of  the  heavens.  It  has  also  been  explained 

;hat  what   corresponds  to  latitude   on   the 
:elestial   sphere   is   called    declination,    and 


Celestial  Latitude  and  Longitude    49 

what  corresponds  to  longitude  is  called 
right  ascension.  It  happens,  however,  that 
these  same  terms,  latitude  and  longitude, 
are  also  employed  in  astronomy.  But,  un- 
fortunately, they  are  based  upon  a  differ- 
ent set  of  circles  from  that  which  has  been 
described,  and  they  do  not  correspond  in 
the  way  that  right  ascension  and  declination 
do  to  terrestrial  longitude  and  latitude.  A 
few  words  must  therefore  be  devoted  to 
celestial  latitude  and  longitude,  as  distin- 
guished from  declination  and  right  ascen- 
sion. 

Celestial  latitude  and  longitude  then, 
instead  of  being  based  upon  the  equator  and 
the  poles,  are  based  upon  the  ecliptic  and 
the  poles  of  the  ecliptic.  Celestial  latitude 
means  distance  north  or  south  of  thejecliptic 
(not  of  the  equator),  and  celestial  longitude 
means  distance  from  the  vernal  equinox 
reckoned  along  the  ecliptic  (not  along  the 
equator).  Celestial  longitude  runs,  the 
same  as  right  ascension,  from  west  toward 
east,  but  it  is  reckoned  in  degrees  instead 
of  hours.  Celestial  latitude  is  measured 
the  same  as  declination,  but  along  circles 
running  through  the  poles  of  the  ecliptic 
instead  of  the  celestial  poles,  and  drawn 


50  The  Celestial  Sphere 

perpendicular  to  the  ecliptic  instead  of  to 
the  equator.  Circles  of  celestial  latitude 
are  drawn  parallel  to  the  ecliptic  and  cen- 
tring round  the  poles  of  the  ecliptic,  and 
meridians  of  celestial  longitude  are  drawn 
through  the  poles  of  the  ecliptic  and  per- 
pendicular to  the  ecliptic  itself.  The 
meridian  of  celestial  longitude  that  passes 
through  the  two  equinoxes  is  the  ecliptic 
prime  meridian.  This  intersects  the  equi- 
noctial colure  at  the  equinoctial  points, 
making  with  it  an  angle  of  23^^  The 
solstitial  colure,  which  it  will  be  remembered 
runs  round  the  celestial  sphere  half-way 
between  the  equinoxes,  is  perpendicular 
to  the  ecliptic  as  well  as  to  the  equator,  and 
so  is  common  to  the  two  systems  of  circles. 
It  passes  alike  through  the  celestial  poles 
and  the  poles  of  the  ecliptic.  It  will  also 
be  observed  that  the  vernal  equinox  is  com- 
mon to  the  two  systems  of  co-ordinates,  be- 
cause it  lies  at  one  of  the  intersections  of  the 
ecliptic  and  the  equator.  In  passing  from  one 
system  to  the  other,  the  astronomer  employs 
the  methods  of  spherical  trigonometry. 

10.  The  Zodiac  and  the  Precession  of  the 
Equinoxes.  The  next  thing  with  which  we 
must  make  acquaintance  is  the  zodiac. 


THe  Zodiac  and  Equinoxes      51 


Fig.  4.     The  Ecliptic  and  Celestial  Latitude  and  Longitude. 

C,  as  in  the  other  figures,  is  the  place  of  the  observer  and 
Z  is  tb|  zenith,  but  to  avoid  complication  of  details  the 
circleJpf  the  horizon  is  not  drawn,  only  the  north-and- 
soutfnine,  N  C  S,  being  shown. 
Eq  |£q'  is  the  equator. 
E|KEc'  is  the  ecliptic. 
P  and  P'  are  the  celestial  poles, 
p  and  p'  are  the  poles  of  the  ecliptic. 
Na  imhe  nadir. 

V  is  nie  jjlrnal  equinox,  and  A  the  autumnal  equinox. 
The  cf^cle*  through  s,  parallel  to  the  ecliptic,  is  a  latitude 
circle. 

The  cir]pS5jD  s  p'  is  the  ecliptic  meridian  of  the  star  s. 
The  cimeMjfcjP'  A  is  the  equinoctial  colure. 
The  circle  frvp'  A  is  the  prime  ecliptic  meridian. 
The  arc  offehe  ecliptic  meridian  contained  between  the 
ecliptic  and  admeasures  the  star's  latitude. 
The  arc  of^fiie  ecliptic  contained  between  V  and  the 
point    where    the    ecliptic    meridian    p  s  p'    meets    the 
ecliptic  (oix  the  angle  V  p  s)  measures  the  star's  longi- 
tude east  from  V,  the  vernftl  equinox. 


52  THe  Celestial  SpHere 

We  have  learned  that  the  ecliptic  is  a  great 
circle  of  the  celestial  sphere  inclined  at  an 
angle  of  23^/2°  to  the  equator,  and  crossing 
the  latter  at  two  opposite  points  called  the 
equinoxes,  and  that  the  sun  in  its  annual 
journey  round  the  sky  follows  the  circle  of 
the  elliptic.  Consequently,  tfie  place  which 
the  sun  otKupies  at  any  time  must  be 
somewhere  on  the  course  of  the  ecliptic. 
The  fact  has  been  rentioned  that  as  seen 
from  the  sun  the  eSth  would  appear  to 
travel  rotftid  the  ecliptic,  whence  the  ecliptic 
may  be  regarded  as  the  projection  of  the 
earth's  orbit,  or  path,  against  the  back- 
ground of  the  heavens.  But,  besides  the 
earth  there  are  seven  other  large  planets, 
Mercury,  Venus,  Mars,  Jupiter,  Saturn, 
Uranus,  and  Neptune,  which,  like  it,  revolve 
round  the  sun,  some  nearer  and  some  father 
away.  Now,  the  orbits  of  all  of  these  planets 
lie  in  planes  nearly  coincident  with  |Jiat  of 
the  earth's  orbit.  None  of  them  is  inclined 
more  than  7°  from  the  ecliptic  tftd  most 
of  them  are  inclined  only  one  o%  t^o  degrees. 
Consequently,  as  we  watch  th^e  planets 
moving  slowly  round  in  their  orbits  we  find 
that  they  are  always  quite  close  to  the 
circle  of  the  ecliptic.  *  This  fact  shows  that 


THe  Zodiac  and  Equinoxes      53 

the  solar  system,  i.  e.,  the  sun  and  its  attend- 
ant planets,  occupies  a  disk-shaped  area 
in  space,  the  outlines  of  which  would  be 
like  those  of  a  very  thin  round  cheese,  with 
the  sun  in  the  centre.  The  ecliptic  indicates 
the  median  plane  of  this  imaginary  disk. 
The  moon,  too,  travels  nearly  in  this  common 
plane,  its  orbit  round  the  earth  rjeing  inclined 
only  a  little  more  than  5°  to  the  ecliptic. 

Even  the  early  ast^nomers  noticed  these 
facts,  and  in  ancient  times  they  gave  to  the 
apparent  road  round  the  sky  in  'which  the 
sun  and  planets  travel,  in  tracks  relatively 
as  close  together  as  the  parallel  marks  of 
wheels  on  a  highway,  the  name  zodiac. 
They  assigned  to  it  a  certain  arbitrary  width, 
sufficient  to  include  the  orbits  of  all  the 
planets  known  to  them.  This  width  is 
8°  on  each  side  of  the  circle  of  the  ecliptic, 
or  1 6°  in  all.  They  also  divided  the  ring  of 
the  aodiac  into  twelve  equal  parts,  corre- 
sponding with  the  number  of  months  in  a 
year,  arid  each  part  was  called  a  sign  of  the 
zodiac.  $mce  there  are  360°  in  a  circle, 
each  sign  of  the  zodiac  has  a  length  of  just 
30.°  To  indicate  the  course  of  the  zodiac 
to  the  eye,  its  inventors  observed  the  con- 
stellations lying  along  it,  assigning  one 


54  THe  Celestial  SpKere 

constellation  to  each  sign.  Beginning  at 
the  vernal  equinox,  and  running  round  east- 
ward, they  gave  to  these  zodiacal  constel- 
lations, as  well  as  to  the  corresponding  signs, 
names  drawn  from  fancy  resemblances  of 
the  figures  formed  by  the  stars  to  men, 
animals,  or  other  objects.  The  first  sign 
and  constellation  were  called  Aries,  the 
Ram,  indicated  by  the  symbol  T;  the 
second,  Taurus,  the  ,Bull,  tf ;  the  third, 
Gemini,  the  Twins,  K ;  the  fourth,  Cancer, 
the  Crab,  ® ;  the  fifth,  Leo,  the  Lion,  £  ;  the 
sixth,  Virgo,  the  Virgin,  TIP;  the  seventh, 
Libra,  the  Balance,  ^ ;  the  eighth,  Scorpio, 
the  Scorpion,  TTl;  the  ninth,  Sagittarius,  the 
Archer,  ^ ;  the  tenth,  Capricornus,  the 
Goat,  V3 ;  the  eleventh,  Aquarius,  the  Water- 
Bearer,  vz ;  and  the  twelfth,  Pisces,  the 
Fishes,  X.  The  name  zodiac  comes  from  a 
Greek  word  for  animal,  since  most  of  the 
imaginary  figures  formed  by  the  stars  of 
the  zodiacal  constellations  are  those  of 
animals.  The  signs  and  their  corresponding 
constellations  being  supposed  fixed  in  the 
sky,  the  planets,  together  with  the  sun  and 
the  moon,  were  observed  to  run  through  them 
in  succession  from  west  to  east. 

When  this  system  was  invented,  the  signs 


THe  Zodiac  and  E-qxiinoxes      55 

and  their  constellations  coincided  in  posi- 
tion, but  in  the  course  of  time  it  was 
found  that  they  were  drifting  apart,  the  signs, 
whose  starting  point  remained  the  vernal 
equinox,  backing  westward  through  the  sky 
until  they  became  disjoined  from  their 
proper  constellations.  At  present  the  sign 
Aries  is  found  in  the  constellation  next  west 
of  its  original  position,  viz.,  Pisces,  and  so  on 
round  the  entire  circle.  This  motion,  as 
already  intimated,  carries  the  equinoxes 
along  with  the  signs,  so  that  the  vernal 
equinox,  which  was  once  at  the  beginning 
of  the  constellation  Aries  (as  it  still  is  at 
the  beginning,  or  "first  point,"  of  the  sign 
Aries),  is  now  found  in  the  constellation 
Pisces. 

To  explain  the  shifting  of  the  signs  of  the 
zodiac  on  the  face  of  the  sky  we  must 
consider  the  phenomenon  known  >as  the 
precession  of  the  equinoxes,  which  is  one  of 
the  most  interesting  things  in  astronomy. 
Let  us  refer  again  to  the  fact  that  the  axis 
of  the  earth's  daily  rotation  is  inclined  23^/2° 
from  a  perpendicu  ar  to  the  plane  of  its 
yearly  revolution  round  the  sun,  from  which 
it  results  that  the  ecliptic  is  tipped  at  the 
same  angle  to  the  plane  of  the  equator. 


56  Tlie  Celestial  SpKere 

Thus  the  sun,  moving  in  the  elliptic,  appears 
half  the  year  above  (or  north  of)  the  equator, 
and  half  the  year  below  (or  south  of)  it, 
the  crossing  points  being  the  two  equinoxes. 
Now,  this  inclination  of  the  earth's  axis 
is  the  key  to  the  explanation  we  are  seeking. 
The  direction  in  which  the  axis  lies  in  space 
is  a  fixed  direction,  which  can  be  changed  only 
by  some  outside  force  interfering.  What 
we  mean  by  this  will  become  clearer  if  we 
think  of  the  earth's  axis  as  resembling  the 
peg  of  a  top,  or  the  axis  of  a  gyroscope. 
When  a  top  is  spinning  smoothly,  with  its 
peg  vertical,  the  peg  will  remain  vertical  as 
long  as  the  spin  is  not  diminished,  and  no 
outside  force  interferes.  So,  too,  the  axis  of 
the  spinning-wheel  of  a  gyroscope  keeps 
pointing  in  the  same  direction  so  persistently 
that  the  wheel  is  kept  from  falling.  If  it 
is  so  mounted  that  it  is  free  to  move  in  any 
direction,  and  if  then  you  take  the  instru- 
ment in  your  hand  and  turn  round  with  jt, 
the  axis  will  adjust  itself  in  such  a  manner 
as  to  retain  its  original  direction  in  space. 
This  tendency  of  a  rotating  body  to  keep 
its  axis  of  rotation  fixed  applies  equally  to 
the  earth,  whose  axis,  also,  maintains  a 
constant  direction  in  space,  except  for  a  slow 


THe  Zodiac  and  ILqxiinoxes      57 

change  produced  by  outside  forces,  which 
change  constitutes  the  phenomenon  of  the 
precession  of  the  equinoxes. 

We  cannot  too  often  recall  the  fact  that 
the  axis  of  the  earth  is  coincident  in  direction 
with  that  of  the  celestial  sphere,  so  that  the 
earth's  poles  are  situated  directly  under  the 
celestial  poles.  But  the  poles  of  the  ecliptic 
are  23^/2°  aside  from  the  celestial  poles. 
If  the  direction  of  the  earth's  axis  and 
with  it  that  of  the  celestial  sphere,  did 
not  change  at  all,  then  the  celestial  poles 
and  the  poles  of  the  ecliptic  would  always 
retain  the  same  relative  positions  in  the  sky; 
but  -  in  fact,  an 'exterior  force,  acting  upon 
-  -the-learth,  causes  a  gradual  change  in  the 
direction  of  its  axis,  and  in  consequence  of 
this  change  the  celestial  poles,  whose  position 
depends  upon  that  of  the  earth's  poles, 
have  a  slow  motion  of  revolution  about  the 
poles  of  the  ecliptic,  in  a  circle  of  23^/2° 
radius.  The  force  which  produces  this  effect 
is  the  attraction  of  the  sun  and  the  moon 
unon  the  protuberant  part  of  the  earth 
round  its  equator.  If  the  earth  were  a 
perfect  sphere,  this  force  could  not  act, 
or  would  not  exist,  but  since  the  earth  is  an 
oblate  spheroid,  slightly  flattened  at  the 


58  THe  Celestial  Sphere 

poles,  and  bulged  round  the  equator,  the 
attraction  acts  upon  the  equatorial  protuber- 
ance in  such  a  way  as  to  strive  to  pull  the 
earth's  axis  into  an  upright  position  with 
respect  to  the  plane  of  the  ecliptic.  But, 
in  consequence  of  its  spinning  motion,  the 
earth  resists  this  pull,  and  tries,  so  to  speak, 
to  keep  the  inclination  of  its  axis  unchanged. 
The  result  is  that  the  axis  swings  slowly  round 
while  maintaining  nearly  the  same  inclination 
to  the  plane  of  the  ecliptic. 

Here,  again,  we  may  employ  the  illustration 
of  a  top.  If  the  peg  of  the  top  is  tipped  a 
little  aside,  so  that  the  attraction  of  gravi- 
tation would  cause  the  top  to  fall  flat  on  the 
table  if  it  were  not  spinning,  it  will,  as  long 
as  it  continues  to  spin,  swing  round  and  round 
in  a  circle  instead  of  falling.  We  cannot 
enter  into  a  mathematical  explanation  of 
this  phenomenon  here,  but  the  reader  will 
find  a  clear  popular  account  of  the  whole 
matter  in  Prof.  John  Perry's  little  book 
on  Spinning  Tops.  It  is  sufficient  here 
to  say  that  the  attraction  of  gravitation, 
tending  to  make  the  top  fall,  but  really 
causing  the  peg  to  turn  round  and  round, 
resembles,  in  its  effect,  the  attraction  of  the 
sun  and  the  moon  upon  the  equatorial 


The  Milky  Way  about  Chi  Cygni 

Photographed  at  the  Lick  Observatory  by  E.  E.  Barnard,  with  the  six-inch 

Willard  lens. 
Observe  the  cloud-like  forms. 


THe  Zodiac  and  Equinoxes       59 

protuberance  of  the  earth,  which  makes  the 
earth's  axis  turn  round  in  space. 

Now,  as  we  have  said,  this  slow  swinging 
round  of  the  axis  of  the  earth  produces  the 
so-called  precession  of  the  equinoxes.  In  a 
period  of  about  ^^Sop^yeajs,  the  axis  makes 
one  complete  swing  round,  so  that  in  that 
space  of  time  the  celestial  poles  describe 
a  revolution  about  the  poles  of  the  ecliptic, 
which  remain  fixed.  But  since  the  equator 
is  a  circle  situated  half-way  between  the  poles, 
it  is  evident  that  it  must  turn  also.  To 
illustrate  this,  take  a  round  flat  disk  of  tin, 
or  pasteboard,  to  represent  the  equator  and 
its  plane,  and  perpendicularly  through  its 
centre  run  a  straight  rod  to  represent  the 
axis.  Put  one  end  of  the  axis  on  the  table, 
and,  holding  it  at  a  fixed  inclination,  turn 
the  upper  end  round  in  a  circle.  You 
will  see  that  as  the  axis  thus  revolves,  the 
disk  revolves  with  it,  and  if  you  imagine  a 
plane,  parallel  to  the  surface  of  the  table, 
passing  through  the  centre  of  the  disk  at 
the  point  where  the  rod  pierces  it,  you  will 
perceive  that  the  two  opposite  points,  where 
the  edge  of  the  disk  intersects  this  imaginary 
plane,  revolve  with  the  disk.  In  one  position 
of  the  axis,  for  instance,  these  points  may  lie 


60  The  Celestial  Sphere 

in  the  direction  of  the  north  and  south  sides 
of  the  room.  When  you  have  revolved  the 
axis,  and  with  it  the  disk,  one  quarter  way 
round,  the  points  will  lie  toward  the  east 
land  west  sides  of  the  room.  When  you  have 
produced  a  half  revolution  they  will  once 
more  lie  toward  the  north  and  south,  but 
now  the  direction  of  the  slope  of  the  disk 
will  be  the  reverse  of  that  which  it  had  at 
the  beginning.  Finally,  when  the  revolution 
is  completed,  the  two  points  will  again  lie 
north  and  south  and  the  slope  of  the  disk 
will  be  in  the  same  direction  as  at  the  start. 
In  this  illustration  the  disk  stands  for  the 
plane  of  the  celestial  equator,  the  rod  for 
the  axis  of  the  celestial  sphere,  the  imaginary 
plane  parallel  to  the  surface  of  the  table  for 
the  plane  of  the  ecliptic,  and  the  two  opposite 
points  where  this  plane  is  intersected  by  the 
edge  of  the  disk  for  the  equinoxes.  The 
motion  of  these  points  as  the  inclined  disk 
revolves  represents  the  precession  of  the 
equinoxes.  This  term  means  that  the 
direction  of  the  motion  of  the  equinoxes, 
as  they  shift  their  place  on  the  ecliptic, 
is  such  that  they  seem  to  precess,  or  move 
forward,  as  if  to  meet  the  sun  in  its  annual 
journey  round  the  ecliptic.  The  direction 


THe  Zodiac  and  E-qxiinoxes       61 

is  from  east  to  west,  and  thus  the  zodiacal 
signs  are  carried  farther  and  farther  westward 
from  the  constellations  originally  associated 
with  them;  for  these  signs,  as  we  have  said, 
are  so  arranged  that  they  begin  at  the  vernal 
equinox,  and  if  the  equinox  moves,  the 
whole  system  of  signs  must  move  with  it. 
The  _amourit_oi ..  ^h^-J^aQi^Qn^s^^^L-^o'' .  2 
per  year,  and  since  there  are  1,296,000" 
in  a  circle,  simple  division  shows  that  the 
time  required  for  one  complete  revolution  of 
the  equinoxes  must  be,  as  already  stated  in 
reference  to  the  poles,  about  25,800  years. 
A  little  over  2000  years  ago  the  signs  and  the 
constellations  were  in  accord;  it  follows, 
then,  that  about  23,800  years  in  the  future, 
they  will  be  in  accord  again.  In  the  mean- 
while the  signs  will  have  backed  entirely 
round  the  circle  of  the  ecliptic. 

The  attentive  reader  will  perceive  that 
the  precession  of  the  equinoxes,  with  its 
attendant  revolution  of  the  celestial  poles 
round  the  poles  of  the  ecliptic,  must  affect 
the  position  of  the  North  Star.  We  have  al- 
ready said  that  that  star  only  happens  to 
occupy  its  present  commanding  position 
in  the  sky.  The  star  itself  is  motionless, 
or  practically  so,  with  regard  to  the  earth, 


62  THe  Celestial  SpHere 

and  it  is  the  north  pole  that  changes  its  place. 
At  the  present  time  the  pole  is  about  i°  10, 
from  the  North  Star,  in  the  direction  of  the 
Great  Dipper,  and  it  is  slowly  drawing  nearer 
so  that  in  about  200  years  it  will  be  less  than 
half  a  degree  from  the  star.  After  that  the 
precessional  motion  will  carry  the  pole  in  a 
circle  departing  farther  and  farther  from  the 
star,  until  the  latter  will  have  entirely  lost 
its  importance  as  a  guide  to  the  position  of 
the  pole.  It  happens,  however,  that  several 
other  conspicuous  stars  lie  near  this  circle. 
One  of  these  is  Thuban,  or  Alpha  Draconis 
(not  now  as  bright  as  it  once  was),  and  this 
star  at  the  time  when  it  served  as  an  indicator 
of  the  place  of  the  pole,  some  4600  years  ago, 
was  connected  with  a  very  romantic  chapter 
in  the  history  of  astronomy.  In  the  great 
pyramid  of  Cheops  in  Egypt,  there  is  a  long 
passage  leading  straight  toward  the  north 
from  a  chamber  cut  deep  in  the  rock  under 
the  centre  of  the  pyramid,  and  the  upward 
slope  of  this  passage  is  such  that  it  is  believed 
to  have  been  employed  by  the  Egyptian 
astronomer-priests  as  a  kind  of  telescope- 
tube  for  viewing  the  then  pole  star,  and 
observing  the  times  of  its  passage  over  the 
meridian — for  even  the  North  Star,  since  it 


THe  Zodiac  and  Eqviinoxes      63 

is  not  exactly  at  the  pole,  revolves  every 
twenty-four  hours  in  a  tiny  circle  about  it, 
and  consequently  crosses  the  meridian  twice 
a  day,  once  above  and  once  beneath  the  true 
pole. 

About  11,500  years  in  the  future,  the 
extremely  brilliant  star  Vega,  or  Alpha 
Lyras,  will  serve  as  a  pole  star,  although 
it  will  not  be  as  near  the  pole  as  the  North 
Star  now  is.  At  that  time  the  North  Star 
will  be  nearly  50°  from  the  pole.  In  about 
21,000  years  the  pole  will  have  come  round 
again  to  the  neighbourhood  of  Alpha 
Draconis,  the  star  of  the  pyramid,  and  in 
about  25,800  years  the  North  Star  will  have 
been  restored  to  its  present  prestige  as  the 
apparent  hub  of  the  heavens. 

One  curious  irregularity  in  the  motion 
of  the  earth's  poles  must  be  mentioned  in 
connection  with  the  precession  of  the  equi- 
noxes. This  is  a  kind  of  "nodding,"  known 
as  nutation.  It  arises  from  variation  in 
the  effect  of  the  attraction  of  the  sun  and 
the  moon,  due  to  the  varying  directions  in 
which  the  attraction  is  exercised.  As  far 
as  the  sun  is  concerned,  the  precession  is 
slower  near  the  time  of  the  equinoxes  than 
in  other  parts  of  the  year;  in  other  words, 


64  THe  Celestial  SpKere 

it  is  most  rapid  in  mid-summer  and  mid- 
winter when  one  or  the  other  of  the  poles 
is  turned  sunward.  A  similar,  but  much 
larger,  change  takes  place  in  the  effect  of  the 
moon's  attraction  owing  to  the  inclination 
of  her  orbit  to  the  ecliptic.  During  about  nine 
and  a  half  years,  or  half  the  period  of  revolu- 
tion of  her  nodes  (see  Part  III,  Section  4),  the 
moon  tends  to  hasten  the  precession,  and 
during  the  next  nine  and  a  half  years  to 
retard  it.  The  general  effect  of  the  combi- 
nation of  these  irregularities  is  to  cause  the 
earth's  poles  to  describe  a  slightly  waving 
curve  instead  of  a  smooth  circle  round  the 
poles  of  the  ecliptic.  There  are  about 
1400  of  these  " waves,"  or  "nods,"  in  the 
motion  of  the  poles  in  the  course  of  their 
26,ooo-year  circuit.  In  accurate  observation 
the  astronomer  is  compelled  to  take  account 
of  the  effects  of  nutation  upon  the  apparent 
places  of  the  stars. 

A  very  remarkable  general  consequence 
of  the  change  in  the  direction  of  the  earth's 
axis  will  be  mentioned  when  we  come  to. 
deal  with  the  seasons. 


The  Great  Southern  Star-Cluster  w  Centauri 

Photographed  by  S.  I.  Bailey  at  the  South  American  Station  of 

Harvard  Observatory. 

Note  the  streaming  of  small  stars  around  the  cluster.  The  cluster  itself 
is  globular  and  its  stars  are  too  numerous  to  be  counted,  or  even  to  be  sepa- 
rately distinguished  in  the  central  part. 


PART  II. 

THE   EARTH. 


PART  II. 

THE  EARTH. 

i.     Nature,  Shape,  and  Size  of  the  Earth. 

The  situation  of  the  earth  in  the  universe 
has  been  briefly  described  in  Part  I;  it 
remains  now  to  see  what  the  earth  is  in  itself, 
and  what  are  some  of  the  principal  phenom- 
ena connected  with  it  as  a  celestial  body  in- 
habited by  observant  and  reasoning  beings. 
We  know  by  ordinary  experience  that  the 
earth  is  composed  of  rock,  sand,  soil,  etc., 
and  generally  covered,  where  there  is  no 
running  or  standing  water  in  the  form  of 
rivers,  lakes,  or  seas,  with  vegetation,  such 
as  trees  and  grass.  Further  experience 
teaches  us  that  the  earth  is  very  large,  and 
that  its  surface  is  divided  into  wide  areas 
of  land  and  of  water.  The  largest  bodies  of 
water,  the  oceans,  taken  all  together,  cover 
about  72  per  cent.,  or  nearly  three-quarters 
of  the  entire  surface  of  the  earth.  Investi- 
gations carried  as  far  down  as  we  can  go 
show  that  the  interior  of  the  earth  consists 
67 


68  TKe  EartK 

of  various  kinds  of  rock,  in  which  are  con- 
tained many  different  kinds  of  metals.  While 
there  is  reason  for  thinking  that  a  high 
degree  of  temperature  prevails  deep  in  the 
earth,  yet  it  appears  evident,  for  other 
reasons,  that,  taken  as  a  whole,  it  is  solid 
and  very  rigid  throughout.  By  methods, 
the  history  and  description  of  which  we  have 
not  here  sufficient  space  to  give,  it  has  been 
proved  that  the  earth  is,  in  form,  a  globe, 
or  more  strictly  an  ellipsoid,  slightly  drawn 
in  at  the  poles  and  swollen  round  the  equator. 
The  polar  diameter  is  7899  miles,  and  the 
equatorial  diameter  7926  miles,  the  difference 
amounting  to  only  27  miles.  Thus,  for 
ordinary  purposes,  we  may  regard  the  earth 
as  being  a  true  sphere.  The  level  of  its 
surface,  however,  is  varied  by  hills  and 
mountains,  which,  though  insignificant  in 
comparison  with  the  size  of  the  whole  earth, 
are  enormous  when  compared  with  the 
structures  of  human  hands.  The  loftiest 
known  mountain  on  the  earth,  Mt.  Everest 
in  the  Himalayas,  has  an  elevation  of  29,000 
feet  above  sea-level,  and  the  deepest  known 
depression  of  the  ocean  bottom,  near  the 
island  of  Guam  in  the  Pacific,  sinks  31,614 
feet  below  sea-level.  Thus,  the  apex  of  the 


The  EartK  69 

highest  mountain  is  about  eleven  and  a  half 
miles  in  vertical  elevation  above  the  bottom 
of  the  deepest  pit  of  the  sea — a  distance  very 
considerably  less  than  half  the  difference 
between  the  equatorial  and  polar  diameters 
of  the  earth. 

It  is  believed  that  at  the  beginning  of 
its  history  the  earth  was  a  molten  mass, 
or  perhaps  a  mass  of  hot  gases  and  vapours 
like  the  sun,  and  that  it  assumed  its  present 
shape  in  obedience  to  mechanical  laws,  as 
it  cooled  off*  The  rotation  caused  it  to 
swell  round  the  equator  and  draw  in  at  the 
poles. 

The  outer  part  of  the  earth  is  called  its 
crust,  and  geology  shows  that  this  has  been 
subject  to  violent  changes,  such  as  upheavals 
and  subsidences,  and  that  in  many  places 
sea  and  land  have  interchanged  places, 
probably  more  than  once.  Geology  also 
shows  that  the  rocks  of  the  earth's  crust 
are  filled  with  the  remains,  or  fossils,  of 
plants  and  animals  differing  from  those  now 
existing,  though  related  to  them,  and  that 
many  of  these  must  have  lived  millions  of 
years  ago.  Thus  we  see  that  the  earth  bears 
marks  of  an  immense  antiquity,  and  that 
it  was  probably  inhabited  during  vast  ages 


70  The  EartK  , 

before  the  race  of  man  had  been  developed. 
The  origin  of  life  upon  the  earth  is  unknown. 
2.  The  Attraction  of  Gravitation.  Among 
the  phenomena  of  life  upon  the  earth,  which 
are  so  familiar  that  only  thoughtful  persons 
see  anything  to  wonder  at  in  them,  is  what 
we  call  the  "weight"  of  bodies.  Every 
person  feels  that  he  is  held  down  to  the 
ground  by  his  weight,  and  he  knows  that  if 
he  drops  a  heavy  body  it  will  fall  straight 
toward  the  ground.  But  what  is  this  weight 
which  causes  everything  either  to  rest  upon 
the  earth  or  to  fall  back  to  it  if  lifted  up  and 
dropped?  The  answer  to  this  question  in- 
volves a  principle,  or  "law,"  which  affects 
the  whole  universe,  and  makes  it  what  we 
see  it.  This  principle  is  one  of  the  great 
foundation  stones  of  astronomy.  It  is 
called  the  law  of  gravitation,  the  word  gravi- 
tation being  derived  from  the  Latin  gravis, 
"heavy."  Briefly  stated,  the  law  is  that 
every  body,  or  every  particle  of  matter,  at- 
tracts, or  strives  to  draw  to  itself,  every  other 
body,  or  particle  of  matter.  This  force  is  called 
the  attraction  of  gravitation.  A  large  body 
possesses  more  attractive  force  than  a 
small  one,  in  proportion  to  the  mass,  or 
quantity  of  matter,  that  it  contains.  The 


The  Attraction  of  Gravitation      71 

earth,  being  extremely  large,  holds  all  bodies 
on  its  surface  with  a  force  proportionate 
to  its  great  mass.  This  explains  why  we 
possess  what  we  call  weight,  which  is  simply 
the  effect  of  the  attraction  of  the  earth  upon 
our  bodies.  A  large  body  is  heavier,  or 
drawn  with  more  force  by  the  earth,  than  a 
small  one  (composed  of  the  same  kind  of 
matter),  because  it  has  a  greater  mass. 
The  body  really  attracts  the  earth  as  much 
as  the  earth  attracts  the  body,  but  the 
amount  of  motion  caused  by  the  attraction 
is  proportional  to  the  respective  masses  of 
the  attracting  bodies,  and  since  the  mass  of 
the  earth  is  almost  infinitely  great  in  com- 
parison with  that  of  any  body  that  we  can 
handle,  the  motion  which  the  latter  imparts 
to  the  earth  is  imperceptible,  and  it  is  the 
small  body  only  that  is  seen  to  move  under 
the  force  of  the  attraction. 

Now  we  are  going  to  see  how  vastly 
important  in  its  effects  is  the  fact  that  the 
earth  is  spherical  in  form.  Sir  Isaac  Newton, 
who  first  worked  out  mathematically  the 
law  of  gravitation,  proved  that  a  spherical 
body  attracts,  and  is  attracted,  as  if  its 
entire  mass  were  concentrated  in  a  point  at 
its  centre.  From  this  it  follows  that  the 


72  The  EartK 

attraction  of  the  earth  is  exercised  just  as  if 
the  whole  attractive  force  emanated  from 
a  middle  point,  and,  that  being  so,  the  effect 
of  the  attraction  is  to  draw  bodies  from  all 
sides  toward  the  centre  of  the  earth.  This 
explains  why  people  on  the  opposite  side  of 
the  earth,  or  under  our  feet,  as  we  say, 
experience  the  same  attractive  force,  or 
have  the  same  weight,  that  we  do.  All 
round  the  earth,  no  matter  where  they  may 
be  situated,  objects  are  drawn  toward  the 
centre.  If  at  any  point  on  the  earth  you 
suspend  a  plumb-line,  and  then,  going  one 
quarter  way  round,  suspend  another  plumb- 
line,  each  of  the  lines  will  be  vertical  at 
the  place  where  it  hangs,  and  yet,  the 
directions  of  the  two  lines  will  be  at  right 
angles  to  one  another,  since  both  point  toward 
the  centre  of  the  earth. 

Knowing  the  manner  in  which  the  earth 
attracts,  we  have  the  means  of  determining 
its  entire  mass,  or,  as  it  is  sometimes  called, 
the  weight  of  the  earth.  The  principle 
on  which  this  is  done  is  easily  understood, 
Suppose,  for  instance,  that  a  small  ball  of 
lead,  of  known  weight,  is  brought  near  a 
large  ball,  and  delicately  suspended  in  such 
a  way  that,  by  microscopic  observation,  the 


THe  Attraction  of  Gravitation      73 

movement  imparted  by  the  attraction  of 
the  large  ball  can  be  measured.  The  force 
required  to  produce  this  movement  can  be 
compared  with  the  force  of  the  earth's 
attraction  which  produces  the  weight  of 
the  ball,  and  thus  the  ratio  \  of  the  mass 
of  the  earth  to  that  of  the  l^all  is  deter- 
mined. The  total  mass  of  thh  earth  has 
been  found  to  be  equivalent  to  a\"  weight" 
of  about  6,5OO,ooo,ooo,ooo,ooo,ooo,bio  tons. 
The  mean  density  of  the  earth  compared  with 
that  of  water  is  found  to  be  about  5^/2,  that 
is  to  say,  the  earth  weighs  5^/2  times  as  much 
as  a  globe  of  water  of  equal  size. 

Newton  did  not  stop  with  showing  the 
manner  of  the  earth's  attraction  upon  bodies 
on  or  near  its  surface;  he  proved  that  the 
earth  attracted  the  moon  also,  and  thus 
retained  it  in  its  orbit.  To  understand  this 
we  must  notice  another  fact  concerning 
the  manner  in  which  gravitation  acts.  Its 
force  varies  with  distance.  Experiment 
followed  by  mathematical  demonstration, 
has  proved  that  the  variation  of  the  attrac- 
tion is  inversely  proportional  to  the  square 
of  the  distance.  This  simply  means  that  if 
the  distance  between  the  two  bodies  con- 
cerned is  doubled,  the  force  of  attraction  will 


74  The  Earth 

be  diminished  four  times,  4  being  the  square 
of  2 ;  and  that  if  the  distance  is  halved,  the 
force  will  be  increased  fourfold.  Increase 
the  distance  three  times,  and  the  force 
diminishes  nine  times;  diminish  the  distance 
three  times,  and  the  force  increases  nine 
times,  because  9  is  the  square  of  3,  and,  as 
we  have  said,  the  force  varies  inversely, 
or  contrarily,  to  the  change  of  distance. 
Knowing  this,  Newton  computed  what  the 
force  of  the  earth's  attraction  must  be  on 
the  moon,  and  he  found  that  it  was  just 
sufficient  to  keep  the  latter  moving  round 
and  round  the  earth.  But  why  does  not 
the  moon  fall  directly  to  the  earth?  Because 
the  moon  had  originally  another  motion 
across  the  direction  of  the  earth's  attraction. 
How  it  got  this  motion  is  a  question  into 
which  we  cannot  here  enter,  but,  if  it  were 
not  attracted  by  the  earth  (or  by  the  sun), 
the  moon  would  travel  in  a  straight  line 
through  space,  like  a  stone  escaping  from  a 
sling.  The  force  of  the  attraction  is  just 
sufficient  to  make  the  moon  move  in  an 
orbital  path  about  the  earth. 

The  same  principle  was  extended  by 
Newton  to  explain  the  motion  of  the  earth 
around  the  sun.  The  force  of  the  sun's 


THe  Attraction  of  Gravitation     75 

attraction,    calculated    in    the    same    way, 
can  be  shown  to  be  just  sufficient  to  retain 


Fig.  5.     How  the  Earth  Controls  the  Moon. 

Let  C  be  the  centre  of  the  earth  and  M  that  of  the  moon. 
Suppose  the  moon  to  be  moving  in  a  straight  line  at  such 
a  velocity  that  it  will,  if  not  interfered  with,  go  to  A  in 
one  day.  Now  suppose  the  attraction  of  the  earth  to 
act  upon  it.  That  attraction  will  draw  it  to  M'.  Again 
suppose  that  at  M'  the  moon  were  suddenly  released 
from  the  earth's  attraction;  it  would  then  shoot  straight 
ahead  to  B  in  the  course  of  the  next  day.  But,  in  fact, 
the  earth's  attraction  acts  continually,  and  in  the  second 
day  the  moon  is  drawn  to  M*.  In  other  words  the 
moon  is  all  the  time  falling  away  from  the  straight  line 
that  it  would  pursue  but  for  the  earth's  attraction,  and 
yet  it  does  not  get  nearer  the  earth  but  simply  travels 
in  an  endless  curve  round  it. 

the  earth  in  its  orbit  and  prevent  it  from 


76  THe  EartK 

travelling  away  into  space.  And  so  with 
all  the  other  planets  which  revolve  round 
the  sun.  And  this  applies  throughout  the 
universe.  There  are  certain  so-called  double, 
or  binary,  stars,  which  are  so  close  together 
that  their  attraction  upon  one  another  causes 
them  to  revolve  in  orbits  about  their  common 
centre.  In  truth,  all  the  stars  attract  the 
earth  and  the  sun,  but  the  force  of  this 
attraction  is  so  slight  on  account  of  their 
immense  distance  that  we  cannot  observe 
its  effects.  The  reader  Who  wishes  to  pursue 
this  subject  of  gravitational  attraction  should 
consult  more  extensive  works,  such  as  Prof. 
Young's  General  Astronomy,  or  Sir  George 
Airy's  Gravitation. 

3.  The  Tides.  The  tides  in  the  ocean 
are  a  direct  result  of  the  attraction  of  gravi- 
tation. They  also  involve  in  an  interesting 
way  the  principle  that  a  spherical  body,  like 
the  earth,  attracts  and  is  attracted  as  if  its 
entire  mass  were  concentrated  at  its  centre. 
The  cause  of  the  tides  is  the  difference  in 
the  attraction  of  the  sun  and  moon  upon  the 
body  of  the  earth  as  a  rigid  sphere,  and  upon 
the  water  of  the  oceans,  as  a  fluid  envelope 
whose  particles,  while  not  free  to  escape  from 
the  earth,  are  free  to  move,  or  slide,  among 


Photograph  of  a  Group  of  Sun-spots 

Similar  groups  are  frequently  seen  during  periods  of  sun-spot  maximum. 


TKe  Tides  77 

one  another  in  obedience  to  varying  forces. 
The  difference  of  the  force  of  attraction 
arises  from  the  difference  of  distance.  Since 
the  moon,  because  of  her  relative  nearness, 
is  the  chief  agent  in  producing  tides  we  shall, 
at  first,  consider  her  tidal  influence  alone. 
The  diameter  of  the  earth  is,  in  round 
numbers,  8000  miles;  therefore,  its  radius 
is  4000  miles.  From  this  it  follows  that  the 
centre  of  the  earth  is  4000  miles  farther  from 
the  moon  than  that  side  of  the  earth  which 
is  toward  her  at  any  time,  and  4000  miles 
nearer  than  the  side  which  is  away  from  her. 
Consequently,  her  attraction  must  be  stronger 
upon  the  water  of  the  ocean  lying  just  under 
her  than  upon  the  centre  of  the  earth,  and  it 
must  also  be  stronger  upon  the  centre  of  the 
earth  than  upon  the  water  of  the  ocean 
lying  upon  the  side  which  is  farthest  from 
her.  The  result  of  these  differences  in  the 
force  of  the  moon's  attraction  is  that  the 
water  directly  under  her  tends  away  from 
the  centre  of  the  earth,  while,  on  the  other 
hand,  the  earth,  considered  as  a  solid  sphere, 
tends  away  from  the  water  on  the  side  op- 
posite to  that  where  the  moon  is,  and  these 
combined  tendencies  cause  the  water  to 
rise,  with  regard  to  its  general  level,  in  two 


78  The  Earth 

protuberances,  situated  on  opposite  sides  of 
the  earth.     These  we  call  tides. 

Some  persons,  when  this  statement  is 
made,  inquire:  "Why,  then,  does  not  the 
moon  take  the  water  entirely  away  from  the 
earth?"  The  answer  is,  that  the  effect  of 
the  tidal  force  is  simply  to  diminish  very 
slightly  the  weight  of  the  water,  or  its  ten- 
dency towards  the  earth's  centre,  but  not 
to  destroy,  or  overmaster,  the  gravitational 
control  of  the  earth.  The  water  retains 
nearly  all  its  weight,  for  the  tidal  force  of 
the  moon  diminishes  it  less  than  one  part 
in  8,000,000.  Still,  this  slight  diminution 
is  sufficient  to  cause  the  water  to  swell  a 
little  above  its  general  level,  at  the  points 
where  it  feels  the  effect  of  the  tidal  force. 
On  the  other  hand,  around  that  part  of  the 
earth  which  is  situated  half-way  between  the 
two  tides,  or  along  a  diameter  at  right  angles 
to  the  direction  of  the  moon,  the  latter's 
attraction  increases  the  weight  of  the  water, 
i.  e.,  its  tendency  toward  the  earth's  centre 
(see  Fig.  6).  Perhaps  this  can  better  be 
understood,  if  we  imagine  the  earth  to  be 
entirely  liquid.  In  that  case  the  difference 
in  the  force  of  the  moon's  attraction  with 
difference  of  distance  would  be  manifested 


The  Tides  79 

in  varying  degrees  throughout  the    earth's 
whole  frame,  and  the  result  would  be  to  draw 


Fig.  6.     The  Tidal  Force  of  the  Moon. 

The  solid  earth  is  represented  surrounded  by  a  shell  of 
water.  The  water  on  the  side  toward  the  moon  is  more 
attracted  than  the  centre  of  the  earth,  C ;  the  water  on  the 
opposite  side  is  less  attracted.  The  lines  of  force  from 
the  moon  to  the  parts  of  the  water  lying  toward  A  and 
B  are  inclined  to  the  direct  line  between  the  centres  of 
the  earth  and  the  moon,  and  the  forces  acting  along 
these  lines  tend  to  draw  the  water  in  the  directions 
shown  by  the  arrow  points.  These  are  resultants  of  the 
horizontal  and  vertical  components  of  the  moon's 
attraction  at  the  corresponding  points  on  the  earth,  and 
the  force  acting  along  them  tends  to  increase  the  weight 
of  the  water  wherever  the  lines  are  inclined  more  toward 
the  centre  of  the  earth  than  toward  the  moon.  On  the 
side  opposite  the  moon  the  same  effects  are  produced  in 
reverse,  because  on  that  side  the  general  tendency  is  to 
draw  the  earth  away  from  the  water.  Consequently  if 
the  earth  did  not  rotate,  and  if  it  were  surrounded  with  a 
complete  shell  of  water,  the  latter  would  be  drawn  into 
an  ellipsoidal  shape,  with  the  highest  points  under  and 
opposite  to  the  moon,  and  the  lowest  at  the  extremities 
of  the  diameter  lying  at  right  angles  to  the  direction  of 
the  moon. 

the    watery    globe    out    into    an   ellipsoidal 
figure,  having  its  greatest  diameter  in  the 


8o  THe  Earth 

line  of  the  moon's  attraction,  and  its  smallest 
diameter  at  right  angles  to  that  line.  The 
proportions  of  the  ellipsoid  would  be  such 
that  the  forces  would  be  in  equilibrium. 

Owing  to  a  variety  of  causes,  such  as  the 
rotation  of  the  earth  on  its  axis,  which  carries 
the  water  rapidly  round  with  it ;  the  inertia 
of  the  water,  preventing  it  from  instantly 
responding  -to  the  tidal  force;  the  irregular 
shape  of  the  oceans,  interrupted  on  all  sides 
by  great  areas  of  land;  their  varying  depth, 
producing  differences  of  friction,  and  so  on, 
the  tidal  waves  do  not  appear  directly  under, 
or  directly  opposite  to,  the  moon,  and  the 
calculation  of  the  course  and  height  of  the 
actual  tides,  at  particular  points  on  the  earth, 
becomes  one  of  the  most  difficult  problems  in 
astronomical  physics. 

We  now  turn  to  consider  the  effects  of  the 
sun's  tidal  force  in  connection  with  that  of 
the  moon.  This  introduces  further  compli- 
cations. The  solar  tides  are  only  about 
two-fifths  as  high  as  the  lunar  tides,  but  they 
suffice  to  produce  notable  effects  when  they 
are  either  combined  with,  or  act  in  opposition 
to,  the  others.  They  are  combined  twice  a 
month — once  when  the  moon  is  between  the 
earth  and  the  sun,  at  the  time  of  new  moon; 


The  TidU  j  81 

and  again  when  the  moon  is  in  opposition 
to  the  sun,  at  the  time  of  full  moon.  In 
these  two  positions  the  attractions  of  the  sun 
and  the  moon  must,  so  to  speak,  act  together, 
with  the  result  that  the  tides  produced  by 
them  blend  into  a  single  greater  wave.  This 
combination  produces  what  are  called  spring 
tides,  the  highest  of  the  month.  When,  on 
the  other  hand,  the  moon  is  in  a  position  at 
right  angles  to  the  direction  of  the  sun, 
which  happens  at  the  lunar  phases  named 
first  and  last  quarters,  the  solar  and  the 
lunar  tides  have  their  crests  90°  apart, 
and,  in  a  sense,  act  against  one  another, 
and  then  we  have  the  neap  tides,  which  are 
the  lowest  of  the  month. 

Without  entering  into  a  demonstration, 
it  may  here  be  stated  as  a  fact  to  be  memor- 
ised, jthat  the  tidal  force  exerted  by  any 
celestial  body  varies  inversely  as  the  cube 
of  the  distance}  This  is  the  reason  why  the 
sun,  although  it  exceeds  the  moon  in  mass 
more  than  25,000,000  times,  and  is  situated 
only  about  400  times  as  far  away  from  the 
earth,  exercises  comparatively  so  slight  a 
tidal  force  on  the  water  of  the  ocean.  If  the 
tidal  force  varied  as  the  square  of  the  dis- 
tance, like  the  ordinary  effects  of  gravitation, 

6 


82  '   He  Earth 

the  tides  produced  by  the  sun  would  be  more 
than  150  times  as  high  as  those  produced 
y  the  moon,  and  would  sweep  New  York, 
London,  and  all  the  seaports  of  the  world 
to  destruction.  In  that  case  it  might  be 
possible,  by  delicate  observations,  to  detect 
a  tidal  effect  produced  upon  the  oceans  of 
the  earth  by  the  planet  Jupiter. 

4.  The  Atmosphere.  The  solid  globe  of 
the  earth  is  enveloped  in  a  mixture  of  gases, 
principally  oxygen  and  nitrogen,  which  we 
call  the  air,  or  the  atmosphere,  and  upon 
whose  presence  our  life  and  most  other 
forms  of  life  depend.  The  atmosphere 
is  retained  by  the  attraction  of  the  earth, 
and  it  rotates  together  with  the  earth. 
If  this  were  not  so — if  the  atmosphere 
stood  fast  while  the  earth  continued  to  spin 
within  it — a  terrific  wind  would  constantly 
blow  from  the  east,  having  a  velocity  at  the 
equator  of  more  than  a  thousand  miles  an 
hour. 

Exactly  how  high  the  atmosphere  extends 
we  do  not  know — it  may  not  have  any 
definite  limits — but  we  do  know  that  its 
density  rapidly  diminishes  with  increase 
of  height  above  the  ground,  so  that  above 
an  elevation  of  a  few  miles  it  becomes  so  rare 


THe  .AtmospHere  83 

that  it  would  not  support  human  life.  The 
phenomena  of  meteors,  set  afire  by  the  fric- 
tion of  their  swift  rush  through  the  upper 
air,  prove,  however,  that  there  is  a  perceptible 
atmosphere  at  an  elevation  of  more  than  a 
hundred  miles. 

From  an  astronomical  point  of  view,  the 
most  important  effect  of  the  presence  of  the 
atmosphere  is  its  power  of  refracting  light. 
By  refraction  is  meant  the  property  possessed 
by  every  transparent  medium  of  bending, 
under  particular  circumstances,  the  rays  of 
light  which  enter  it  out  of  their  original 
course.  The  science  of  physics  teaches  us 
that  if  a  ray  of  light  passes  from  any  trans- 
parent medium  into  another  which  is  denser, 
and  if  the  path  of  this  ray  ^  not  perpen- 
dicular to  the  surface  of  the  second  medium, 
it  will  be  turned  from  its  original  course 
in  such  a  way  as  to  make  it  more  nearly 
perpendicular.  Thus,  if  a  ray  of  light  passes 
from  air  into  water  at  a  certain  slope  to  the 
surface,  it  will,  upon  entering  the  water, 
be  so  changed  in  direction  that  the  slope 
will  become  steeper.  Only  if  it  falls  per- 
pendicularly upon  the  water  will  it  continue 
on  without  change  of  direction.  Conversely 
a  ray  passing  from  a  denser  into  a  rarer 


84  THe  EartK 

medium  is  bent  away  from  a  perpendicular 
to  the  surface  of  the  first  medium,  or  its 
slope  becomes  less.  This  explains  why,  if 
we  put  a  coin  in  a  bowl,  with  the  eye  in  such 
a  position  that  it  cannot  see  the  coin  over 
the  edge,  and  then  fill  the  bowl  with  water, 
the  coin  seems  to  be  lifted  up  into  sight. 
Moreover,  if  any  transparent  medium  in- 
creases in  density  with  depth,  the  amount 
of  refraction  will  increase  as  the  ray  goes 
deeper,  and  the  direction  of  the  ray  will  be 
changed  from  a  straight  line  into  a  curve, 
tending  to  become  more  and  more  perpen- 
dicular. 

Now  all  this  applies  to  the  atmosphere. 
If  a  star  is  seen  in  the  zenith,  its  light  falls 
perpendicularly  into  the  atmosphere  and 
its  course  is  not  deviated,  or  in  other  words 
there  is  no  refraction.  But  if  the  star  is 
somewhere  between  the  zenith  and  the 
horizon,  its  light  falls  slopingly  into  the 
atmosphere,  and  is  subject  to  refraction, 
the  amount  of  bending  increasing  with 
approach  to  the  horizon.  Observation  shows 
that  the  refraction  of  the  atmosphere,  which 
is  zero  at  the  zenith,  increases  to  about  half 
a  degree  (and  sometimes  much  more,  depend- 
ing upon  the  state  of  the  air),  near  the 


The  Atmosphere  85 

horizon.  It  follows  that  a  celestial  object 
seen  near  the  horizon  will  ordinarily  appear 
about  half  a  degree  above  its  true  place. 
Since  the  apparent  diameters  of  the  sun 
and  the  moon  are  about  half  a  degree,  when 


Fig.  7.     Refraction. 

Suppose  an  observer  situated  at  O  on  the  earth.  The  sun, 
at  S,  has  sunk  below  the  level  of  his  horizon,  O  H,  but 
since  the  sun  sends  out  rays  in  all  directions  there  will 
be  some,  such  as  S  A  B,  which  will  strike  the  atmosphere 
at  A,  and  the  refraction,  tending  to  make  the  ray  more 
nearly  perpendicular  to  the  surface  of  the  atmosphere, 
will,  instead  if  allowing  it  to  go  on  straight  over  the 
observer's  head  to  B,  bend  it  down  along  the  dotted  line 
A  O,  and  the  observer  will  see  the  sun  as  if  it  lay  in  the 
directic^of  the  dotted  line  O  A  S',  which  places  the  sun 
appar^tly  above  the  horizon. 

they  are  rising  or  setting  they  can  be  seen 
on  the  horizon  before  they  have  really  risen 
above  it,  or  after  they  have  really  sunk 
below  it.  Tables  of  refraction  at  various 
altitudes  have  been  prepared,  and  they  have 
to  be  consulted  in  all  exact  observations  of 
the  celestial  bodies. 


86  The  Earth 

5.  Dip  of  the  Horizon.  Another  correc- 
tion which  has  to  be  applied  in  many 
observations  depends  upon  the  sphericity 
of  the  earth.  We  have  described  the  rational 
horizon,  and  pointed  out  how  it  differs  from 
the  sensible  horizon.  We  have  also  said 
that  at  sea  the  sensible  horizon  nearly 
accords  with  the  rational  horizon  (see 
Part  I,  Sect.  3).  But  the  accord  is  not 
complete,  owing  to  what  is  called  the  dip 
of  the  horizon.  In  fact,  the  sea  horizon  lies 
below  the  rational  horizon  by  an  amount 
varying  with  the  elevation  of  the  eye  above 
the  surface.  Geometry  enables  us  to  deter- 
mine just  what  the  dip  of  the  horizon  must 
be  for  any  given  elevation  of  the  eye.  A 
rough  and  ready  rule,  which  may  serve  for 
many  purposes,  is  that  the  square  root  of 
the  elevation  of  the  eye  in  feet  equals  the 
dip  of  the  horizon  in  minutes  of  arc,  or  of 
angular  measure.  The  reader  will  readily 
see  that  the  dip  of  the  horizon  is  a  necessary 
consequence  of  the  rotundity  of  the  earth. 
It  is  because  of  this  that,  as  a  ship  recedes 
at  sea  her  hull  first  disappears  below  the 
horizon,  and  then  her  lower  sails,  and  finally 
her  top-sails.  The  use  of  a  telescope  does 
not  help  the  matter,  because  a  telescope 


Dip  of  tHe  Horizon  87 

only   sees    straight,    and    cannot    bend    the 
line  of  sight  over  the  rim  of  the  horizon. 


Fig.  8.     Dip  of  the  Horizon. 

It  is  to  be  remembered  that  it  is  the  sensible  horizon 
which  dips,  and  not  the  rational  horizon.  The  sensible 
horizon  of  the  observer  at  the  elevation  A  dips  below  the 
horizontal  plane  and  he  sees  round  the  curved  surface 
as  far  as  a;  in  other  words  his  skyline  is  at  a.  The 
observer  at  the  elevation  B  has  a  sensible  horizon  still 
more  inclined  and  he  sees  as  far  as  b.  If  the  observation 
were  made  from  an  immense  height  the  observer  would 
see  practically  half  round  the  earth  just  as  we  see  half 
round  the  globe  of  the  moon. 

Atmospheric  refraction,  however,  enables  us 
to  see  an  object  which  would  be  hidden  by 
the  horizon  if  there  were  no  air.  In  navi- 


88  The  Earth 

gation,  which,  as  a  science,  is  an  outgrowth 
of  astronomy,  these  things  have  to  be  care- 
fully taken  into  account. 

6.  Aberration.  A  few  words  must  be  said 
about  the  phenomenon  known  as  aberration 
of  light.  This  is  an  apparent  displacement 
of  a  celestial  object  due  to  the  motifti  of 
the  earth  in  its  orbit.  It  is  customary  to 
illustrate  it  by  imagining  oneself  to  be  in  a 
shower  of  rain,  whose  drops  are  falling 
vertically.  In  such  a  case,  if  a  person 
stands  fast  the  rain  will  descend  perpen- 
dicularly upon  his  head,  but  if  he  advances 
rapidly  in  any  direction  he  will  feel  the  drops 
striking  him  in  the  face,  because  his  own 
forward  motion  is  compounded  with  the 
downward  motion  of  the  rain  so  that  the 
latter  seems  to  be  descending  slantingly 
toward  him.  The  same  thing  happens  with 
the  light  falling  from  the  stars.  As  the 
earth  advances  in  its  orbit  it  seems  to  meet 
the  light  rays,  and  they  appear  to  come  from 
a  direction  ahead  of  the  flying  earth.  The 
result  is  that,  since  we  see  a  star  in  the 
direction  from  which  its  light  seems  to  come, 
the  star  appears  in  advance  of  its  real 
position,  or  of  the  position  in  which  we 
would  see  it  if  the  earth  stood  fast.  The 


Polar  Streamers  of  the  Sun,  Eclipse  of  1889 


The  Solar  Corona  at  the  Eclipse  of  1871 

From  drawings. 


Aberration  89 

amount  by  which  the  position  of  a  star 
is  shifted  by  aberration  depends  upon  the 
ratio  of  the  earth's  velocity  to  the  velocity 
of  light.  In  round  numbers  this  ratio  is  as 
i  to  10,000.  The  motion  of  the  earth  being 
in  a  slightly  eccentric  ellipse,  the  stars 
describe  corresponding,  but  very  tiny,  ellipses 
once  every  year  upon  the  background  of  the 
sky.  But  the  precise  shape  of  the  ellipse 
depends  upon  the  position  of  the  star  on 
the  celestial  sphere.  If  it  is  near  one  of  the 
poles  of  the  ecliptic,  it  will  describe  an 
annual  ellipse  which  will  be  almost  a  circle, 
its  greater  diameter  being  41"  of  arc.  If  it 
is  near  the  plane  of  the  ecliptic,  it  will 
describe  a  very  eccentric  ellipse4  but  the 
greater  diameter  will  always  be  41 ",  although 
the  shorter  diameter  may  be  immeasurably 
small.  The  effects  of  aberration  have  to 
be  allowed  for  in  all  careful  astronomical 
observation  either  of  the  sun  or  the  stars. 
This  is  done  by  reducing  the  apparent 
place  of  the  object  to  the  place  it  would  have 
if  it  were  seen  at  the  centre  of  its  annual 
ellipse. 

7.  Time.  Without  astronomical  obser- 
vations we  could  have  no  accurate  knowledge 
of  time.  The  basis  of  the  measurement  of 


90  The  EartK 

time  is  furnished  by  the  rotation  of  the  earth 
on  its  axis.  We  divide  the  period  which  the 
earth  occupies  in  making  one  complete 
turn  into  twenty-four  equal  parts,  or  hours. 
The  ascertainment  of  this  period,  called  a 
day,  depends  upon  observations  of  the  stars. 
Suppose  we  see  a  certain  star  exactly  on  the 
meridian  at  some  moment;  just  twenty-four 
hours  later  that  star  will  have  gone  entirely 
round  the  sky,  and  will  again  appear  on  the 
meridian.  The  revolving  heavens  constitute 
the  great  clock  of  clocks,  by  whose  movements 
all  other  clocks  are  regulated.  We  know 
that  it  is  not  the  heavens  which  revolve, 
but  the  earth  which  rotates,  but  for  conven- 
ience we  accept  the  appearance  as  a  substitute 
for  the  fact.  The  rotation  of  the  earth  is 
so  regular  that  no  measurable  variation  has 
been  found  in  two  thousand  years.  We  have 
reasons  for  thinking  that  there  must  be  a 
very  slow  and  gradual  retardation,  owing 
principally  to  the  braking  action  of  the  tides, 
but  it  is  so  slight  that  we  cannot  detect 
it  with  any  means  at  present  within  our 
command. 

In  Part  I  it  was  shown  how  the  passage 
across  the  meridian  of  the  point  in  the  sky 
called  the  vernal  equinox  serves  to  indicate 


Time  91 

the  beginning  of  the  astronomical  "day," 
but  the  position  of  the  vernal  equinox  itself 
has  to  be  determined  by  observations  on  the 
stars.  By  means  of  a  telescope,  so  mounted 
that  it  can  only  move  up  or  down,  round  a 
horizontal  axis,  and  with  the  axis  pointing 
exactly  east  and  west  so  that  the  up  and  down 
movements  of  the  telescope  tube  follow  the 
line  of  the  meridian,  the  moment  of  passage 
across  the  meridian  of  a  star  at  any  altitude 
can  be  observed.  Observations  of  this  nature 
are  continually  made  at  all  great  government 
observatories,  such  as  the  observatory  at 
Washington  or  that  at  Greenwich,  and  at 
many  others,  and  by  their  means  clocks  and 
chronometers  are  corrected,  and  a  standard 
of  time  is  furnished  to  the  whole  world. 

There  are,  however,  three  different  ways 
of  reckoning  time,  or,  as  it  is  usually  said, 
three  kinds  of  time.  One  is  sidereal  time, 
which  is  indicated  by  the  passage  of  stars 
across  the  meridian,  and  which  measures  the 
true  period  of  the  earth's  rotation;  another 
is  apparent  solar  time,  whichjs  indicated  by 
the  passage  of  the  sun  across  the  meridian; 
ancTlf  third'  JLS_ mean  solar  time,  which  is 
indicateoT" by  a  carefully  regulated  clock, 
whose  errors  are  corrected  by  star  ob- 


92  The  EartK 

servations.  This  last  kind  of  time  is__that 
wTiich  is  universally  used  in  ordinary  fife" 
(the  use  of  sidereal  time  being  confined  to 
astronomy),  so  it  is  necessary  to  explain 
what  it  is  and  how  it  differs  from  apparent 
solar  time. 

In  the  first  place,  the  reason  why  sidereal 
time  is  not  universally  and  exclusively  used 
is  because,  although  it  measures  the  true 
period  of  the  earth's  rotation  by  the  apparent 
motion  of  the  stars,  it  does  not  exactly  accord 
with  the  apparent  motion  of  the  sun;  and, 
naturally,  the  sun,  since  it  is  the  source  of 
light  for  the  earth,  and  the  cause  of  the  dif- 
ference between  day  and  night,  is  taken 
for  all  ordinary  purposes,  as  the  standard 
indicator  of  the  progress  of  the  hours.  The 
fact  that  it  is  mid-day,  or  noon,  at  any  place 
when  the  sun  crosses  the  meridian  of  that 
place,  is  a  fact  of  common  knowledge,  which 
cannot  be  ignored.  On  the  other  hand,  the 
vernal  equinox,  which  is  the  ''noon  mark" 
for  sidereal  time,  is  independent  of  the 
alternation  of  day  and  night,  and  may  be 
on  the  meridian  as  well  at  midnight  as  at 
mid-day.  Before  clocks  and  watches  were 
perfected,  the  moment  of  the  sun's  passage 
over  the  meridian  was  determined  by  means 


Time  93 

of  a  gnomon,  which  shows  the  instant  of 
noon  by  the  length  of  a  shadow  cast  by  an 
upright  rod.  Since  the  apparent  course  of 
the  sun  through  the  sky  is  a  curve,  rising 
from  the  eastern  horizon,  attaining  its  great- 


Fig.  9.     Sidereal  and  Solar  Time. 

C  is  the  centre  of  the  earth,  and  O  the  place  of  an 
observer  on  the  earth's  surface. 

Suppose  the  sun  at  A  to  be  in  conjunction  with  the  star  S. 
Then,  at  the  end  of  twenty-four  sidereal  hours,  when  the 
earth  has  made  one  turn  on  its  axis  and  the  place  O  has 
again  come  into  conjunction  with  the  star,  the  sun,  in 
consequence  of  its  yearly  motion  in  the  ecliptic,  will 
have  advanced  to  B,  and  the  earth  will  have  to  turn 
through  the  angle  A  C  B  before  O  will  overtake  the  sun 
and  complete  a  solar  day;  wherefore  the  solar  day  is 
longer  than  the  sidereal. 

est  elevation  where  it  meets  the  meridian, 
and  thence  declining  to  the  western  horizon, 
it  is  evident  that  the  length  of  the  shadow 
must  be  least  when  the  sun  is  on  the  meridian, 
or  at  its  maximum  altitude.  The  gnomon, 
or  the  sun-dial,  gives  us  apparent  solar  time. 
But  this  differs  from  sidereal  time  because, 
as  we  saw  in  Part  I,  the  sun,  in  consequence 
of  the  earth's  motion  round  it,  moves 


94  The  Earth 

about  one  degree  eastward  every  twenty- 
four  hours,  and,  since  one  degree  is  equal  to 
four  minutes  of  time,  the  sun  rises  about  four 
minutes  later,  with  reference  to  the  stars, 
every  morning.  Consequently  it  comes  four 
minutes  later  to  the  meridian  day  after  day. 
Or,  to  put  it  in  another  way,  suppose  that 
the  sun  and  a  certain  star  are  upon  the 
meridian  at  the  same  instant.  The  star  is 
fixed  in  its  place  in  the  sky,  but  the  sun  is 
not  fixed ;  on  the  contrary  it  moves  about  one 
degree  eastward  (the  same  direction  as  that 
of  the  earth's  rotation)  in  twenty-four  hours. 
Then,  when  the  "rotation  of  the  earth  has 
brought  the  star  back  to  the  meridian  at  the 
end  of  twejity-four  sidereal  hours,  the  sun, 
in  consequence  of  its  motion,  will  still  be  one 
degree  east  of  the  meridian,  and  the  earth 
must  turn  through  the  space  of  another 
degree,  which  will  take  four  minutes,  before 
it  can  have  the  sun  again  upon  the  meridian. 
The  true  distance  moved  by  the  sun  in 
twenty-four  hours  is  a  little  less  than  one 
degree,  and  the  exact  time  required  for  the 
meridian  to  overtake  it  is  3  min.  56.555  sec. 
Thus,  the  sidereal  day  (period  of  24  hours) 
is  nearly  four  minutes  shorter  than  The  solar 
day. 


• 


Time  95 

It  would  seem,  then,  that  by  taking  the 

sun~"for  a  guide,  and  dividing  the  period 
between  two  of  its  successive  passages  over 
the  meridian  into  twenty-four  hours,  we 
should  have  a  perfect  measure  of  time, 
without  regard  to  the  stars;  in  other  words, 
that  apparent  solar  time  would  be  entirely 
satisfactory  for  ordinary  use.  But,  unfor- 
tunately, the  apparent  eastward  motion  of 
the  sun  is  not  regular.  It  is  sometimes 
greater  than  the  average  and  sometimes  less. 
This  variation  is  due  almost  entirely:  first, 
to  the  fact  that  its  orbit  not  being  a  perfect 
circle  the  earth  moves  faster  when  it  is 
near  perihelion,  and  slower  when  it  is  near 
aphelion;  and,  second,  to  the  effects  of  the 
inclination  of  the  ecliptic  to  the  equator. 
In  consequence,  another  measure  of  solar 
time  is  used,  called  mean  solar  time,  in  which, 
by  imagining  a  fictitious  sun,  moving  with 
perfect  regularity  through  the  ecliptic,  the 
discrepancies  are  avoidecL  All  ordinary 
clocks  are  set  to  follow  this  fictitious,  or 
mean,  sun.  The  result  is  that  clock  time 
does  not  agree  exactly  with  sun-dial  time, 
or,  what  is  the  same  thing,  apparent  solar 
time.  The  clock  is  ahead  of  the  real  sun 
at  some  times  of  the  year,  and  behind  it  at 


96  The  EartH 

other  times.  This  difference  is  called  the 
equation  of  time.  Four  times  in  the  year 
the  equation  is  zero,  i.e.,  there  is  no  difference 
between  the  clock  and  the  sun.  These 
times  are  April  15,  June  14,  Sept.  I,  and 
Dec.  24.  At  four  other  times  of  the  year 
the  difference  is  at  a  maximum,  viz.  Feb.  II, 
sun  14  min.  27  sec.  behind  clock;  May  14, 
sun  3  min.  49  sec.  ahead  of  clock;  July  26, 
sun  6  min.  16  sec.  behind  clock;  Nov.  2, 
sun  1 6  min.  18  sec.  ahead  of •  clock.  These 
dates  and  differences  vary  very  slightly' 
from  year  to  year. 

But,  whatever  measures  of  time  we  may 
use,  it  is  observation  of  the  stars  that  fur- 
nishes the  means  of  correcting  them. 

8.  Day  and  Night.  The  period  of  twenty- 
four  hours  required  for  one  turn  of  the  earth 
on  its  axis  is  called  a  day,  and  in  astronomical 
reckoning  it  is  treated  as  an  undivided  whole, 
the  hours  being  counted  uninterruptedly 
from  o  to  24;  but  nature  has  divided  the 
period  into  two  very  distinct  portions,  one 
characterised  by  the  presence  and  the  other 
by  the  absence  of  the  sun.  Popularly  we 
speak  of  the  sunlighted  portion  as  day  and 
of  the  other  as  night,  and  there  are  no  two 
associated  phenomena  in  nature  more  com- 


Morehouse's  Comet,  October  15,  1908 

Photographed  at  the  Yerkes  Observatory  by  E.  E.  Barnard    with  the  ten- 
inch  Bruce  telescope.     Exposure  one  hour  and  a  half. 
Note  the  detached  portions  which  appeared  to  separate  from  the  head 
and  retreat  up  the  line  of  the  tail  at  enormous  velocity. 


Morehouse's  Comet,  November  15,  1908 

Photographed  at  the  Yerkes  Observatory  by  E.  E.  Barnard,  with  the  ten- 
inch  Bruce  telescope.     Exposure  forty  minutes. 


Day  and  Ni&'Ht  97 

pletely  in  contrast  one  to  the  other.  The 
cause  of  the  contrast  between  day  and  night 
must  have  been  evident  to  the  earliest  human 
beings  who  were  capable  of  any  thought  at 
all.  They  saw  that  day  inevitably  began 
whenever  the  sun  rose  above  the  horizon, 
and  as  inevitably  ceased  whenever  it  sank 
beneath  it.  In  all  literatures,  imaginative 
writers  have  pictured  the  despair  of  primeval 
man  when  he  first  saw  the  sun  disappear 
and  night  come  on,  and  his  joy  when  he  first 
beheld  the  sun  rise,  bringing  day  back  with 
it.  Even  his  uninstructed  mind  could  not 
have  been  in  doubt  about  the  causal  con- 
nection of  the  sun  with  daylight. 

We  now  know  that  the  cause  of  the  alter- 
nate rising  and  setting  of  the  sun,  and  of  its 
apparent  motion  through  the  sky,  is  the 
rotation  of  the  earth.  Making  in  our  minds 
a  picture  of  the  earth  as  a  turning  globe 
exposed  to  the  sunbeams,  we  are  able  to  see 
that  one  half  of  it  must  necessarily  be  il- 
luminated, while  the  other  half  is  in  darkness. 
We  also  see  that  its  rotation  causes  these 
two  halves  gradually  to  interchange  places 
so  that  daylight  progresses  completely  round 
the  earth  once  in  the  course  of  twenty-four 
hours.  If  the  earth  were  not  surrounded 

7 


98  The  Earth 

by  an  atmosphere,  exactly  one  half  of  it 
would  lie  in  the  sunlight  and  exactly  one 
half  in  darkness,  but  the  atmosphere  causes 
the  illuminated  part  slightly  to  exceed  the 
unilluminated  part.  The  reason  for  this  is 
twofold:  first,  because  the  atmosphere,  being 
transparent  and  extending  to  a  considerable 
height  above  the  solid  globe,  receives  rays 
from  the  sun  after  the  latter  has  sunk  below 
the  horizon,  and  these  rays  cause  a  faint  il- 
lumination in  the  sky  after  the  sun  as  viewed 
from  the  surface  of  the  ground  has  disap- 
peared ;  and,  second,  because  the  air  has  the 
property  of  refracting  the  rays  of  light, 
in  consequence  of  which  the  sun  appears 
above  the  horizon  both  a  little  time  before 
it  has  actually  risen  and  a  little  time  after 
it  has  actually  set.  The  faint  illumination 
at  the  beginning  and  the  end  of  the  day 
is  called  twilight.  Its  cause  is  the  reflection 
of  light  from  the  air  at  a  considerable  ele- 
vation above  the  ground.  Observation 
shows  that  evening  twilight  lasts  until  the 
sun  has  sunk  about  18°  below  the  west- 
ern horizon,  while  morning  twilight  begins 
when  the  sun  is  still  18°  below  the  nearest 
horizon.  The  length  of  time  occupied  by 
twilight,  or  its  duration,  depends  upon  the 


Day  and  NigHt  99 

observer's  place  on  the  earth  and  increases 
with  distance  from  the  equator.  The  length 
of  twilight  at  any  particular  place  also  varies 
with  the  seasons. 

It  will  probably  have  occurred  to  the  reader 
that,  since  day  and  night  are  ceaselessly 
chasing  each  other  round  the  globe,  it  must 
be  necessary  to  choose  some  point  of  begin- 
ning, in  order  to  keep  the  regular  succession 
of  the  days  of  the  week.  The  necessity  for 
this  is  evident  as  soon  as  we  reflect  that 
what  is  sunrise  at  one  place  on  the  earth, 
is  sunset  for  a  place  situated  half-way  round, 
on  the  other  side.  To  understand  this  it 
will  be  better,  perhaps,  to  consider  the  phe- 
nomena of  noon  at  various  places.  It  is 
noon  at  any  place  when  the  sun  is  on  the 
meridian  of  that  place.  But  we  have  seen 
that  every  place  has  its  own  meridian; 
consequently,  since  the  sun  cannot  be  on 
the  meridian  of  more  than  one  place  at  a 
time,  each  different  place  (reckoning  east 
and  west,  for,  of  course,  all  places  lying 
exactly  north  or  south  of  one  another  have 
the  same  meridian),  must  have  its  own  local 
noontime.  Since  the  sun  appears  to  move 
round  the  earth  from  east  to  west,  it  will 
arrive  at  the  meridian  of  a  place  lying  east 


ioo  THe  Earth 

of  us  sooner  than  at  our  meridian,  and  it 
will  arrive  at  our  meridian  sooner  than  at 
that  of  a  place  lying  west  of  us.  Thus,  when 
it  is  noon  at  Greenwich,  it  is  about  7  o'clock 
A.M.,  or  five  hours  before  noon,  at  New 
York,  because  the  angular  distance  westward 
round  the  earth's  surface  from  Greenwich 
to  New  York  is,  in  round  numbers,  75°, 
which  corresponds  with.»ffive  hours  of  time, 
there  being  15°  to  every  hour.  At  the  same 
moment  it  will  be  5  o'clock  P.M.,  or  five 
hours  after  noon,  at-  Cashmere,  because 
Cashmere  lies  75°  east  of  Greenwich.  That 
is  to  say,  the  sun  crosses  the  meridian  of 
Cashmere  five  hours  before  it  reaches  the 
meridian  of  Greenwich,  and  it  crosses  the 
meridian  of  Greenwich  five  hours  before  it 
reaches  that  of  New  York.  At  a  place 
half-way  round  the  circumference  of  the 
globe,  i.e.  180°  either  east  or  west  of 
Greenwich,  it  will  be  midnight  at  the  same 
instant  when  it  will  be  mid-day,  or  noon, 
at  Greenwich.  Now  let  us  consider  this 
for  a  moment. 

It  is  customary  to  change  the  name  of 
the  day  at  midnight.  Thus  at  the  stroke 
of  midnight,  anywhere,  Sunday  gives  place 
to  Monday.  Suppose,  then,  that  the  day 


Day  and  Night 


101 


when  we  see  the  sun  on  the  meridian  at 
Greenwich  happens  to  be  Sunday.     Sunday 


6P.M. 


8P.M. 


2A.M 


Fig,  JO.     The  Change  of  Day. 

The  arrows  show  the  direction  in  which  the  earth  turns 
(from  west  to  east).  It  is  always  noon  at  the  place 
which  is  directly  under  the  sun.  Call  it  Sunday  noon 
at  Greenwich,  at  the  top  of  the  circle;  then  it  is  10  A.M. 
Sunday  at  a  point  30°  west  and  2  P.M.  Sunday  at  a  point 
30°  east,  and  so  on.  Exactly  opposite  to  the  noon  point 
it  is  midnight.  By  common  consent  we  change  the 
name  of  the  day,  and  the  date,  at  midnight;  consequently 
it  is  Sunday  midnight  just  east  of  the  vertical  line  at  the 
bottom  of  the  circle  and  Monday  morning  just  west  of 
it.  If  we  cross  that  line  going  westward  we  shall  pass 
directly  from  Sunday  to  Monday,  and  if  we  cross  it 
going  eastward  we  shall  pass  directly  from  Monday  to 
Sunday.  Since,  by  convention,  this  is  a  fixed  line  on 
the  earth's  surface,  the  same  change  will  take  place  no 
matter  what  the  hour  of  the  day  may  be. 

will  then  be,  so  to  speak,  twelve  hours  old 
at    Greenwich,  because  it    began   there  at 


102  The  Earth 

the  preceding  midnight.  Sunday  will  be 
only  seven  hours  old  at  New  York,  where 
it  also  began  at  the  preceding  midnight. 
In  California,  45°,  or  three  hours,  still  farther 
west  than  New  York,  Sunday  will  be  only 
four  hours  old,  since  the  local  time  there 
is  only  four  hours  after  midnight.  Go  on 
over  the  Pacific  Ocean,  until  we  arrive  at 
a  point  1 80°,  or  twelve  hours,  west  of  Green- 
wich. There,  evidently,  Sunday  will  just 
have  been  born,  the  preceding  day,  Saturday, 
having  expired  at  the  stroke  of  midnight. 
Now  if  we  just  step  over  that  line  of  180° 
in  what  day  shall  we  be?  It  cannot  be 
Sunday,  because  Sunday  has  just  begun  on 
the  line  itself.  It  cannot  be  Saturday, 
because  that  would  be  counting  backward. 
Evidently  it  can  be  no  other  than  Monday. 
Let  us  examine  this  a  little  more  closely. 
It  is  Sunday  noon  at  Greenwich.  We  now 
go  round  the  earth  eastward  instead  of 
westward.  At  90°,  or  six  hours,  east  of 
Greenwich,  we  find  that  it  is  6  P.M.  Sunday 
and  at  180°,  or  twelve  hours,  east  of 
Greenwich  we  find  that  it  is  Sunday  midnight, 
or  in  other  words  Monday  morning.  But 
the  line  of  180°  east  of  Greenwich  coincides 
with  the  line  of  180°  west  of  Greenwich, 


Day  and  Nig'Ht  103 

which  we  formerly  approached  from  the 
opposite  direction.  So  we  see  that  we  were 
right  in  concluding  that  in  stepping  over  that 
line  from  the  east  to  the  west  side,  we  were 
passing  frc*n  Sunday  into  Monday.  It  is 
on  that  line  that  each  day  vanishes  and  its 
successor  takes  its  place.  It  is  the  "date- 
line" for  the  whole  earth,  chosen  by  ""the 
common  consent  of  every  civilised  nation, 
just  as  we  have  seen  that  the  meridian  of 
Greenwich  is  the  common  reference  line  for 
reckoning  longitude.  It  lies  entirely  in  the 
Pacific  Ocean,  hardly  touching  any  island, 
and  it  was  chosen  for  this  very  reason, 
because  if  it  ran  over  inhabited  lands,  like 
Europe  or  America,  it  would  cause  endless 
confusion.  Situated  as  it  is,  it  causes  no 
trouble  except  to  sea  captains,  and  very 
little  to  them.  If  a  ship  crosses  the  line 
going  westward  the  captain  jumps  his  log- 
book one  day  forward.  If  it  is,  for  instance, 
Wednesday  noon,  east  of  the  line  he  calls  it 
Thursday  noon,  as  soon  as  he  has  passed  over. 
If  he  is  going  eastward  he  drops  back  a  day 
on  crossing  the  line,  as  from  Thursday  noon 
to  Wednesday  noon.  The  date-line  theo- 
retically follows  the  iSoth  meridian,  but,  in 
fact,  in  order  to  avoid  certain  groups  of 


104  The  Earth 

islands,  it  bends  about  a  little,  while  keeping 
its  general  direction  from  north  to  south. 

9.  The  Seasons.  We  now  recall  again 
what  was  said  in  Part  I,  about  the  inclination 
of  the  ecliptic,  or  the  apparent  path  of  the 
sun  in  the  heavens,  to  the  equator.  Because 
of  this  inclination,  the  sun  appears  half  the 
year  above  the  equator  and  the  other  half 
below  it.  When  it  is  above  the  equator 
for  people  living  in  the  northern  hemisphere, 
it  is  below  the  equator  for  those  living 
in  the  southern  hemisphere,  and  vice  versa. 
This  is  because  observers  on  opposite  sides 
of  the  plane  of  the  equator  look  at  it  from 
opposite  points  of  view.  For  the  northern 
observer  the  celestial  equator  appears  south 
of  the  zenith ;  for  the  southern  observer  it 
appears  north  of  the  zenith,  its  distance 
from  the  zenith,  in  both  cases,  increasing 
with  the  observer's  distance  from  the  equator 
of  the  earth.  If  he  is  on  the  earth's  equator, 
the  celestial  equator  passes  directly  through 
the  zenith.  For  convenience  we  shall  sup- 
pose the  observer  to  be  somewhere  in  the 
northern  hemisphere. 

Let  us  begin  with  that  time  of  the  year 
when  the  sun  arrives  at  the  vernal  equinox. 
This  occurs  about  the  2ist  of  March.  The 


Head  of  the  Great  Comet  of  1861 

From  a  drawing  by  Warren  De  La  Rue. 


Halley's  Comet,  May  5,  1910 

Photographed  at  the  Yerkes  Observatory  by  E    E.  Barnard,  with  the  ten- 
inch  Bruce  telescope. 

Tnis  was  shortly  before  the  passage  of  the  comet  between  the  earth  and 
the  sun,  when  some  think  its  tail  was  thrown  over  us. 


TTKe  Seasons  105 

sun  is  then  perpendicular  over  the  equator, 
daylight- ..-e^ndsl_uninterruptp.d>  from  pole 
to  pole,  and  day  and  night  (neglecting  the 
effects  of  twilight  and  refraction)  are  of 
equal  length  all  over  the  earth.  Everywhere 
there  are  about  twelve  hours  of  daylight 
and  twelve  hours  of  darkness.  This  is  the 
beginning  of  the  astronomical  spring.  As 
time  goes  on,  the  motion  of  the  sun  in  the 
ecliptic  carries  it  eastward  from  the  vernal 
equinox,  and,  at  the  same  time,  owing  to  the 
inclination  of  the  ecliptic,  it  rises  gradually 
higher  above  the  equator,  increasing  its 
northern  declination  slowly,  day  after  day. 
Immediately  the  equality,  of  day  and  night 
ceases,  and  in  the  northern  hemisphere 
the  day  becomes  gradually  longer  in  duration 
than  the  night,  while  in  the  southern  hemi- 
sphere it  becomes  shorter.  Moreover,  be- 
cause the  sun  is  now  north  of  the  equator, 
daylight  no  longer  extends  from  pole  to  pole 
on  the  earth,  but  the  south  pole  is  in  continual 
darkness,  while  the  north  pole  is  illuminated. 
You  can  illustrate  this,  and  explain  to 
yourself  why  the  relative  length  of  day  and 
night  changes,  and  why  the  sun  leaves  one 
pole  in  darkness  while  rising  higher  over  the 
other,  by  suspending  a  small  terrestrial 


io6  The  EartK 

globe  with  its  axis  inclined  about  23^/2°  from 
the  perpendicular,  and  passing  a  lamp 
around  it  in  a  horizontal  plane.  At  two 
points  only  in  its  circuit  will  the  lamp  be 
directly  over  the  equator  of  the  globe.  Call 
one  of  these  points  the  vernal  equinox. 
You  will  then  see  that,  when  the  lamp  is 
directly  over  this  point,  its  light  illuminates 
the  globe  from  pole  to  pole,  but  when  it  has 
passed  round  so  as  to  be  at  a  point  higher 
than  the  equator,  its  light  no  longer  reaches 
the  lower  pole,  although  it  passes  over  the 
upper  one. 

Now,  with  the  lamp  thus  elevated  above 
the  equator,  set  the  globe  in  rotation  about 
its  axis.  You  will  perceive  that  all  points 
in  the  upper  hemisphere  are  longer  in  light 
than  in  darkness,  because  the  plane  dividing 
the  illuminated  and  the  unilluminated  halves 
of  the  globe  is  inclined  to  the  globe's  axis 
in  such  a  way  that  it  lies  beyond  the  upper 
pole  as  seen  from  the  direction  of  the  lamp. 
.Consequently,  the  upper  half  of  the  globe 
above  the  equator,  as  it  goes  round,  has  more 
of  its  surface  illuminated  than  unilluminated, 
and,  as  it  turns  on  its  axis,  any  point  in 
that  upper  half,  moving  round  parallel  to 
the  equator,  is  longer  in  light  than  in  dark- 


THe  Seasons  107 


SL/MMEfl  —  (         )— SUN  '       WINTER 

SOLSTICE  •  *SN*X>  sotsr/cr 


AUTUMN  C.QUINOX. 

Fig.  ii.   .  The  Seasons. 

The  earth  is  represented  at  four  successive  points  in  its 
orbit  about  the  sun.  Since  the  axis  of  the  earth  is 
virtually  unchangeable  in  its  direction  in  space  (leaving 
out  of  account  the  slow  effects  of  the  precession  of  the 
equinoxes),  it  results  that  at  one  time  of  the  year,  the 
north  pole  is  inclined  toward  the  sun  and  at  the  opposite 
time  of  the  year  away  from  it.  It  attains  its  greatest 
inclination  sunward  at  the  summer  solstice,  then  the  line 
between  day  and  night  lies  23^°  beyond  the  north  pole, 
so  that  the  whole  area  within  the  arctic  circle  is  in  per- 
petual daylight.  The  days  are  longer  than  the  nights 
throughout  the  northern  hemisphere,  but  the  day  becomes 
longer  in  proportion  to  the  night  as  the  arctic  circle  is  ap- 
proached, and  beyond  that  the  sun  is  continually  above 
the  horizon.  In  the  southern  hemisphere  exactly  the  re- 
verse occurs.  When  the  earth  has  advanced  to  the  autumn 
equinox,  the  axis  is  inclined  neither  toward  nor  away  from 
the  sun.  The  latter  is  then  perpendicular  over  the  equa- 
tor and  day  and  night  are  of  equal  length  all  over  the  earth. 
When  the  earth  reaches  the  winter  solstice  the  north  pole 
is  inclined  away  from  the  sun,  and  now  it  is  summer  in  the 
southern  hemisphere.  At  the  vernal  equinox  again  there 
is  no-  inclination  of  the  axis  either  toward  or  away  from 
the  sun,  and  once  more  day  and  night  are  everywhere 
equal.  A  little  study  of  this  diagram  will  show  why  on 
the  equator  day  and  night  are  always  of  equal  length. 


io8  The  Earth 

ness.  You  will  also  observe  that  the  ratio 
of  length  of  the  light  to  the  darkness  is 
greater  the  nearer  the  point  lies  to  the  pole, 
and  that  when  it  is  within  a  certain  dis- 
tance of  the  pole,  corresponding  with  the 
elevation  of  the  lamp  above  the  equator, 
it  lies  in  continual  light — in  other  words, 
within  that  distance  from  the  pole  night 
vanishes  and  daylight  is  unceasing.  At  the 
same  time  you  will  perceive  that  round  the 
lower  pole  there  is  a  similar  space  within 
which  day  has  vanished  and  night  is  unceas- 
ing, and  that  in  the  whole  of  the  lower 
hemisphere  night  is  longer  than  day.  Exactly 
on  the  equator,  day  and  night  are  always  of 
equal  length. 

Endeavour  to  represent  all  this  clearly  to 
your  imagination,  before  actually  trying 
the  experiment,  or  consulting  a  diagram. 
If  you  try  the  experiment  you  may,  instead 
of  setting  the  axis  of  the  globe  at  a  slant, 
place  it  upright,  and  then  gradually  raise 
and  lower  the  lamp  as  it  is  carried  round  the 
globe,  now  above  and  now  below  the  equator. 

We  return  to  our  description  of  the  actual 
movements  of  the  sun.  As  it  rises  higher 
from  the  equator,  not  only  does  the  day 
increase  in  length  relatively  to  the  night, 


THe  Seasons  109 

but  the  rays  of  sunlight  descend  more  nearly 
perpendicular  upon  the  northern  hemisphere. 
The  consequence  is  that  their  heating  effect 
upon  the  ground  and  the  atmosphere  increases 
and  the  temperature  rises  until,  when  the 
sun  reaches  its  greatest  northern  declination, 
about  the  226.  of  June  (when  it  is  23^/2° 
north  of  the  equator),  the  astronomical 
summer  begins.  This  point  in  the  sun's 
course  through  the  circle  of  the  ecliptic 
is  called  the  summer  solstice  (see  Part  I, 
Sect.  8).  Having  passed  the  solstice,  the 
sun  begins  to  decline  again  toward  the  equa- 
tor. For  a  short  time  the  declination 
diminishes  slowly  because  the  course  of  the 
ecliptic  close  to  the  solstice  is  nearly  parallel 
to  the  equator,  and  in  the  meantime  the 
temperature  in  the  northern  hemisphere 
continues  t'o  increase,  the  amount  of  heat 
radiated  away  during  the  night  being  less 
than  that  received  from  the  sun  during  the 
day.  This  condition  continues  for  about  six 
weeks,  the  greatest  heats  of  summer  falling 
at  the  end  of  July  or  the  beginning  of  August, 
when  the  sun  has  already  declined  far  toward 
the  equator,  and  the  nights  have  begun 
notably  to  lengthen.  But  the  accumulation 
of  heat  during  the  earlier  part  of  the  summer 


I  io  The  EartK 

is  sufficient  to  counterbalance  the  loss  caused 
by  the  declension  of  the  sun. 

About  the  23d  of  September  the  sun 
again  crosses  the  equator,  this  time  at  the 
autumnal  equinox,  the  beginning  of  the 
astronomical  autumn,  and  after  that  it 
sinks  lower  and  lower  (while  appearing 
to  rise  in  the  southern  hemisphere),  until 
about  the  226.  of  December,  when  it  reaches 
its  greatest  southern  declination,  233/2°,  at 
the  winter  solstice,  which  marks  the  begin- 
ning of  the  astronomical  winter.  It  is 
hardly  necessary  to  point  out  that  the  south- 
ern winter  corresponds  in  time  with  the 
northern  summer,  and  vice  versa.  From 
the  winter  solstice  the  sun  turns  northward 
once  more,  reaching  the  vernal  equinox 
again  on  the  2ist  of  March. 

Thus  we  see  that  we  owe  the  succession 
of  the  seasons  entirely  to  the  inclination 
of  the  earth's  axis  out  of  a  perpendicular  to 
the  plane  of  the  ecliptic.  If  there  were  no 
such  inclination  there  would  be  climate  but 
no  seasons.  There  would  be  no  summer 
heat,  except  in  the  neighbourhood  of  the 
equator,  while  the  middle  latitudes  would 
have  a  moderate  temperature  the  year  round. 
Owing  to  the  effects  of  refraction,  perpetual 


THe  Seasons  ill 

day  would  prevail  within  a  small  region 
round  each  of  the  poles.  The  sun  would  be 
always  perpendicular  over  the  equator. 

Two  things  remain  to  be  pointed  out  with 
regard  to  the  effect  of  the  sun's  annual  motion 
in  the  ecliptic.  One  of  these  is  the  circles 
called  the  tropics.  These  are  drawn  round 
the  earth  parallel  to  the  equator  and  at  a 
distance  of  23^/2°  from  it,  one  in  the  northern 
and  the  other  in  the  southern  hemisphere. 
The  northern  one  is  called  the  tropic  of 
Cancer,  because  its  corresponding  circle 
on  the  celestial  sphere  runs  through  the 
zodiacal  sign  Cancer,  and  the  southern  one 
is  called  the  tropic  of  Capricorn  for  a  similar 
reason.  The  tropics  run  through  the  two 
solstices,  and  mark  the  apparent  daily  track 
of  the  sun  in  the  sky  when  it  is  at  either  its 
greatest  northern  or  its  greatest  southern 
declination.  The  sun  is  then  perpendicular 
over  one  or  the  other  of  the  tropics.  That 
part  of  the  earth  lying  between  the  tropics 
is  called  the  torrid  zone,  because  the  sun  is 
always  not  far  from  perpendicular  over  it, 
and  the  heat  is  very  great. 

The  other  thing  to  be  mentioned  is  the 
polar  circles.  These  are  situated  23^/2° 
from  each  pole,  just  as  the  tropics  are  situated 


H2  The  Earth 

a  similar  distance  on  each  side  of  the  equator. 
The  northern  is  called  the  arctic,  and  the 
southern  the  antarctic  circle.  Those  parts 
of  the  earth  which  lie  between  the  tropics 
and  the  polar  circles  are  called  respectively 
the  northern  and  the  southern  temperate 
zone.  The  polar  circles  mark  the  limits  of 
the  region  round  each  pole  where  the  sun 
shines  continuously  when  it  is  at  one  or  the 
other  of  the  solstices.  If  the  reader  will 
recall  the  experiment  with  the  globe  and  the 
lamp,  he  will  perceive  that  these  circles 
correspond  with  the  borders  of  the  circular 
spaces  at  each  pole  of  the  globe  which  are 
alternately  carried  into  and  out  of  the  full 
light  as  the  lamp  is  elevated  to  its  greatest 
height  above  the  equator  or  depressed  to  its 
eatest  distance  below  it.  At  each  pole, 
in  turn,  there  are  six  months  of  continual 
day  followed  by  six  months  of  continual 
night,  and  when  the  sun  is  at  one  of  the 
solstices  it  just  touches  the  horizon  on  the 
corresponding  polar  circle  at  the  hour  that 
marks  midnight  on  the  parts  of  the  earth 
which  lie  outside  the  polar  circles.  This 
is  the  celebrated  phenomenon  of  the  "  mid- 
night sun."  At  any  point  within  the  polar 
circle  concerned,  the  sun,  at  the  hour  of 


The  Six-Tailed  Comet  of  1744 

From  a  contemporary  drawing. 


THe  Seasons  113 

midnight  approaches  the  horizon  but  does 
not  touch  it,  its  midnight  elevation  in- 
creasing with  nearness  to  the  pole,  while 
exactly  at  the  pole  itself  the  sun  simply  moves 
round  the  sky  once  in  twenty-four  hours 
in  a  circle  practically  parallel  to  the  horizon. 
It  is  by  observations  on  the  daily  movement 
of  the  sun  that  an  explorer  seeking  one 
the  earth's  poles  during  the  long  polar  day 
is  able  to  determine  when  he  has  actually 
reached  his  goal. 

The  reader  will  have  remarked  in  these 
descriptions  how  frequently  the  angle  of 
2 3^/2°  turns  up,  and  he  should  remember  that 
it  is,  in  every  case,  due  to  the  same  cause, 
viz.,  the  inclination  of  the  earth's  axis  from 
a  perpendicular  to  the  ecliptic. 

A  very  remarkable  fact  must  now  be 
referred  to.  Although  the  angular  distance 
that  the  sun  has  to  travel  in  passing  first  frpm 
the  vernal  equinox  to  the  autumnal  equinox, 
on  the  northern  side  of  the  equator,  and  then 
back  again  from  the  autumnal  equinox  to 
the  vernal  equinox,  on  the  southern  side  of 
the  equator,  is  the  same,  the  time  that  it 
occupies  in  making  these  two  half  stages 
in  its  annual  journey  is  not  the  same. 
Beginning  from  the  2ist  of  March  and 


114  The  Earth 

counting  the  number  of  days  to  the  23d 
of  September,  and  then  beginning  from  the 
23d  of  September  and  counting  the  number 
of  days  to  the  next  2ist  of  March,  you  will 
find  that  in  an  ordinary  year  the  -first 
period  is  seven  days  longer  than  the  second. 
In  other  words,  the  sun  is  a  week  longer 
above  the  equator  than  below  it.  The 
reason  for  this  difference  is  found  in  the  fact 
that  the  orbit  of  the  earth  about  the  sun  is 
not  a  perfect  circle,  but  is  a  slightly  elongated 
ellipse,  and  the  sun,  instead  of  being  situated 
in  the  centre,  is  situated  in  one  of  the  two 
foci  of  the  ellipse,  3,000,000  miles  nearer  to 
one  end  of  it  than  to  the  other.  Now  this 
elliptical  orbit  of  the  earth  is  so  situated 
that  the  earth  is  nearest  to  the  focus  occupied 
by  the  sun,  or  in  perihelion,  about  December 
3 1st,  only  a  few  days  after  the  winter  solstice, 
and  farthest  from  the  sun,  or  in  aphelion, 
about  July  1st,  only  a  few  days  after  the 
summer  solstice.  Thus  the  earth  is  nearer 
the  sun  during  the  winter  half  of  the  year, 
when  the  sun  appears  south  of  the  equator, 
than  during  the  summer  half  of  the  year, 
when  the  sun  appears  north  of  the  equator. 
Now  the  law  of  gravitation  teaches  that 
when  the  earth  is  nearer  the  sun  it  must  move 


THe  Seasons  115 

more  rapidly  in  its  orbit  than  when  it  is 
more  distant,  from  which  it  follows  that  the 
time  occupied  by  the  sun  in  its  apparent 
passage  from  the  vernal  equinox  to  the  au- 
tumnal equinox  is  longer  than  that  occupied 
in  the  passage  back  from  the  autumnal  to 
the  vernal  equinox. 

But  while  the  summer  half  of  the  year 
is  longer  than  the  winter  half  in  the  northern 
hemisphere,  the  reverse  is  the  case  in  the 
southern  hemisphere.  There  the  winter  is 
longer  than  the  summer.  Moreover,  the 
winter  of  the  southern  hemisphere  occurs 
when  the  earth  is  farthest  from  the  sun, 
which  accentuates  the  disadvantage.  It  has 
been  thought  that  the  greater  quantity  of 
ice  about  the  south  pole  may  be  due  to  this 
increased  length  and  severity  of  the  southern 
winter.  It  is  true  that  the  southern  summer, 
although  shorter,  is  hotter  than  the  northern, 
but  while,  theoretically,  this  should  restore 
the  balance  as  a  whole,  yet  it  would  appear 
that  the  short  hot  summer  does  not,  in  fact, 
suffice  to  arrest  the  accumulation  of  ice. 

However,  the  present  condition  of  things 
as  between  the  two  hemispheres  will  not 
continue,  but  in  the  course  of  time  will  be 
reversed.  The  reader  will  recall  that  the 


Ii6  The  Earth 

precession  of  the  equinoxes  causes  the  axis 
of  the  earth  to  turn  slowly  round  in  space. 
At  present  the  northern  end  ofHhe  earth's 
axis  is  inclined  away  from  the  aphelion  and 
in  the  direction  of  the  perihelion  point  of  the 
orbit,  so  that  the  northern  summer  occurs 
when  the  earth  is  in  the  more  distant  part 
of  its  orbit,  and  the  winter  when  it  is  in  the 
nearer  part.  But  the  precession  swings  the 
axis  round  westward  from  its  present  position 
at  the  rate  of  5o".2  per  year,  while  at  the 
same  time  the  position  of  the  orbit  itself 
is  shifted  (by  the  effects  of  the  attraction 
of  the  planets)  in  such  a  manner  that  the 
aphelion  and  perihelion  points,  which  are 
called  the  apsides,  move  round  eastward 
at  the  rate  of  n".25  per  year.  The  combina- 
tion of  the  precession  with  the  motion  of 
the  apsides  produces  a  revolution  at  the  rate 
of  6i'/.45  per  year,  which  in  the  course  of 
10,500  years  will  completely  reverse  the 
existing  inclination  of  the  axis  with  regard 
to  the  major  diameter  of  the  orbit,  so  that 
then  the  northern  hemisphere  will  have  its 
summer  when  the  earth  is  near  perihelion 
and  its  winter  when  it  is  near  aphelion. 
The  winter,  then,  will,  for  us,  be  long  and 
severe  and  the  summer  short  though  hot. 


Year,  Calendar,  and  MontK      117 

It  has  been  thought  possible  that  such  a 
state  of  things  may  cause,  in  our  hemisphere, 
a  partial  renewal  of  what  is  known  in  geology 
as  a  glacial  period.  A  glacial  period  in  the 
southern  hemisphere  would  probably  always 
be  less  severe  than  in  the  northern,  because 
of  the  great  preponderance  of  sea  over  land 
in  the  southern  half  of  the  globe.  An  ocean 
climate  is  more  equable  than  a  land  climate. 
10.  The  Year,  the  Calendar,  and  the 
Month.  A  year  is  the  period  of  time  required 
for  the  earth  to  make  one  revolution  in  its 
orbit  about  the  sun.  But,  as  there  are 
three  kinds,  or  measures,  of  time,  so  there  are 
three  kinds,  or  measures,  of  the  year.  The 
first  of  these  is  called  the  sidereal  vjsar,  but 
although,  like  sidereal  time,  it  measures  the 
true  length  of  the  period  in  question,  it  is 
not  suitable  for  ordinary  use.  To  understand 
what  is  meant  by  a  sidereal  year,  imagine 
yourself  to  be  looking  at  the  earth  from  the 
sun,  and  suppose  that  at  some  instant  you 
should  see  the  earth  exactly  in  conjunction 
with  a  star.  When,  having  gone  round  the 
sun,  it  had  come  back  again  to  conjunction 
with  the  same  star,  precisely  one  revolution 
would  have  been  performed  in  its  orbit, 
and  the  period  elapsed  would  be  a  sidereal 


Ii8  The  Earth 

year.  Practically,  the  length  of  the  sidereal 
year  is  determined  by  observing  when  the 
sun,  in  its  apparent  annual  journey  round 
the  sky,  has  come  back  to  conjunction  with 
some  given  star. 

The  second  kind  of  year  is  called  the  tropi- 
cal year,  and  it  is  measured  by  the  period 
taEeif  by  the  sun  to  pass  round  the  sky  from 
one  conjunction  with  the  vernal  equinox  to 
the  next.  This  period  differs  slightly  from  the 
first,  because,  owing  to  the  precession  of  the 
equinoxes,  the  vernal  equinox  is  slowly 
shifting  westward,  as  if  to  meet  the  sun  in  its 
annual  course,  from  which  it  results  that  the 
sun  overtakes  the  equinox  a  little  before 
it  has  completed  a  sidereal  year.  The 
tropical  year  is  about  twenty  minutes  shorter 
than__the  sidergal  year.  It  is,  however, 
more  convenient  for  ordinary  purposes, 
because  we  naturally  refer  the  progress  of 
the  year  to  that  of  the  seasons,  and,  as  we 
have  seen,  the  seasons  depend  upon  the 
equinoxes. 

But  yet  the  tropical  year  is  not  entirely 
satisfactory  as  a  measure  of  time,  because 
the  number  of  days  contained  in  it  is  not  an 
even  one.  Its  length  is  366  days,  5  hours, 
48  minutes,  46  seconds.  Accordingly,  as 


Tear,  Calendar,  and  MontK      119 

the  irregularities  of  apparent  solar  time  were 
avoided  by  the  invention  of  mean  solar  time, 
so  the  difficulty  presented  by  the  tropical 
year  is  gotten  rid  of,  as  far  as  possible,  by 
means  of  what  is  called  the.  .tijdLyear,  or 
the  calendar  .year,  the  average  length  of 
which  is  almost  exactly  equal  to  that  of 
the  tropical  year.  This  brings  us  to  the 
consideration  of  the  calendar,  which  is  as 
full  of  compromises  as  a  political  treaty— 
but  there  is  no  help  for  it  since  nature  did 
not  see  fit  to  make  the  day  an  exact  fraction 
of  the  year,  or,  in  other  words,  to  make  the 
day  and  the  year  commensurable  quantities 
of  time. 

Without  going  into  a  history  of  the  reforms 
that  the  calendar  has  undergone,  which  would 
demand  a  great  deal  of  space,  we  may  simply 
say  that  the  basis  of  the  calendar  we  use  to- 
day was  established  by  Julius  Caesar,  with 
the  aid  of  the  Greek  astronomer  Sosigenes. 
This  is  the  Julian  calendar,  and  the  reformed 
shape  in  which  it  exists  at  present  is  called 
the  Gregorian  calendar.  Caesar  assumed 
365/i  days  as  the  true  length  of  the  year, 
and,  in  order  to  get  rid  of  the  quarter  day, 
ordered  that  it  should  be  left  out  of  account 
for  three  years  out  of  every  four.  In  the 


120  THe  EartK 

fourth  year  the  four  quarter_days  were  added 


together  to  make  one  additional  day, 
which  was  added  to  that  particular  year. 
Thus  the  ordinary  years  were  each  365 
days  long  and  every  fourth  year  was  366 
days  long.  This  fourth  year  was  called 
the  bissextile  year.  It  was  identical  with 
our  leap  year.  The  days  of  both  the  ordin- 
ary and  the  leap  years  were  distributed  among 
the  twelve  months  very  much  as  we  distrib- 
ute them  now. 

But  Caesar's  assumption  of  365^  days 
as  the  length  of  the  year  was  erroneous, 
being  about  umin.  14  sec.  longer  than  the 
real  tropical  year.  In  the  sixteenth  century 
this  error  had  accumulated  to  such  a  degree 
that  the  months  were  becoming  seriously 
disjointed  from  the  seasons  with  which  they 
had  been  customarily  associated.  In  con- 
sequence, Pope  Gregory  XIII,  assisted  by 
the  astronomer  Clavius,  introduced  ja^  slight 
reform  of  the  Julian  calendar.  The  accumu- 
lated  days  were  dropped,  and  a  new  start 
taken,  and  the  rule  for  leap  year  was  changed 
so  as  to  read  that  "all  years,  whose— daie- 
number  is  divisible  by  four  without  a  remain- 
der are  leap  years,  unless  they  are  century 
years  (such  as  1800,  1900,  etc.).  The 


Year,  Calendar,  and  MontH      121 

century  years  are  not  leap  years,  unless  their 
date  number  is  divisible  by  400,  in  which 
case  they  are."  And  this  is  the  rule  as  it 
prevails  to-day,  although  there  is  now 
(1912)  serious  talk  of  undertaking  a  new 
revision.  But  the  Gregorian  calendar  is 
so  nearly  correct  that  more  than  3000  years 
must  elapse  before  the  length  of  the  year 
as  determined  by  it  will  differ  by  one  day 
from  the  true  tropical  year. 

The  subject  of  the  reform  of  the  calendar 
is  a  very  interesting  one,  but,  together  with 
that  of  the  rules  for  determining  the  date  of 
Easter,  its  discussion  must  be  sought  in  more 
extensive  works. 

There  is  one  other  measure  of  time, 
depending  upon  the  motion  of  a  heavenly 
body,  which  must  be  mentioned.  This  is 
the  month,  or  the  period  required  for  the 
moon  to  make  a  revolution  round  the  earth. 
Here  we  encounter  again  the  same  difficulty, 
for  the  month  also  is  incommensurable  with 
the  year.  Then,  too,  the  length  of  the  month 
varies  according  to  the  way  in  which  it  is 
reckoned.  We  have,  first,  a  sidereal  revo- 
lution of  the  moon,  which  is  measured  by  the 
time  taken  to  pass  round  the  earth  from  one 
conjunction  with  a  star  to  the  next.  This 


122  TKe  Earth 

is,  on  the  average,  27  days,  7  hours,  43  min- 
utes, 12  seconds.  Next  we  have  asynodical 
revolution  of  the  moon,  which  is  measured 
by  the  time  it  takes  in  passing  from  the  phase 
of  new  moon  round  to  the  same  phase  again. 
This  seems  the  most  natural  measure  of  a 
month,  because  the  changing  phases  of  the 
moon  are  its  most  conspicuous  peculiarity. 
(These  will  be  explained  in  Part  III.)  The 
length  of  the  month,  as  thus  measured,  is, 
on  the  average,  29  day s,"  12  hours,  44  minutes, 
3  seconds.  The  reason  why  the  synodical 
month  is  so  much  longer  than  the  sidereal 
month  is  because  new  moon  can  occur  only 
when  the  moon  is  in  conjunction  with  the  sun, 
i.e.  exactly  between  the  earth  and  the  sun,  and 
in  the  interval  between  two  new  moons  the 
sun  moves  onward,  so  that  for  the  second  con- 
junction the  moon  must  go  farther  to  overtake 
the  sun.  It  will  be  observed  that  both  of  the 
month  measures  are  given  in  average  figures. 
This  is  because  the  moon's  motion  is  not 
quite  regular,  owing  partly  to  the  eccentricity 
of  its  orbit  and  partly  to  the  disturbing 
effects  of  the  sun's  attraction.  The  length  of 
the  sidereal  revolution  varies  to  the  extent  of 
three  hours,  and  that  of  the  synodical  revo- 
lution to  the  extent  of  thirteen  hours. 


Year,  Calendar,  and  Month.      123 

But,  whichever  measure  of  the  month  we 
take,  it  is  incommensurate  with  the  year, 
i.e.  there  is  not  an  even  number  of  months 
in  a  year.  In  ancient  times  ceaseless  efforts 
were  made  to  adjust  the  months  to  the 
measure  of  the  year,  but  we  have  practically 
given  up  the  attempt,  and  in  our  calendar 
the  lunar  months  shift  along  as  they  will, 
while  the  ordinary  months  are  periods  of  a 
certain  number  of  days,  having  no  relation 
to  the  movements  of  the  moon. 

It  has  been  thought  that  the  period  called 
a  week,  which  has  been  used  from  time 
immemorial,  may  have  originated  from  the 
fact  that  the  interval  from  new  moon  to  the 
first  quarter  and  from  first  quarter  to  full 
moon,  etc.,  is  very  nearly  seven  days.  But 
the  week  is  as  incorrigible  as  all  its  sisters 
in  the  discordant  family  of  time,  and  there 
is  no  more  difficult  problem  for  human  inge- 
nuity than  that  of  inventing  a  system  of 
reckoning,  in  which  the  days,  the  weeks, 
the  months,  and  the  years  shall  be  adjusted 
to  the  closest  possible  harmony. 


Spiral  Nebula  in  Ursa  Major     (M  101) 

Photographed  at  the  Lick  Observatory  by  J.  E.  Keeler,  with  the  Crossley 

reflector.     Exposure  four  hours. 

Note  the  appearance  of  swift  revolution,  as  if  the  nebula  were  throwing 
itself  to  pieces  like  a  spinning  pin-wheel. 


The  Whirlpool  Nebula  in  Canes  Venatici 

Photographed  at  the  Lick  Observatory  by  J.  E.  Keeler,  with  the  Crossley 

reflector.      Exposure  four  hours. 
Note  the  "  beading  "  of  the  arms  of  the  whirling  nebula. 


PART  III. 

THE  SOLAR  SYSTEM, 


125 


PART  III. 

THE  SOLAR  SYSTEM. 

i.  The  Sun.  By  the  term  solar  system 
is  meant  the  sun  together  with  the  system 
of  bodies  (planets,  astef^ids,  comets  and 
meteors)  revolving  round  it.  The  sun,  being 
a  star,  every  other  star,  tor  all  that  we  can 
tell,  may  be  the  ruler  of  a  similar  system. 
In  fact,  we  know  that  a  few  stars  have  huge 
dark  bodies  revolving  round  them,  which 
may  be  likened  to  gigantic  planets.  The 
reason  why  the  sun  is  the  common  centifcr 
round  which  the  other  members  of  the  solar 
system  move,  is  because  it  vastly  exceeds 
all  of  them  put  together  in  mass,  or  quantity 
of  matter,  and  the  power  of  any  body  to  set 
another  body  in  motion  by  its  attractive 
force  depends  upon  mass.  If  a  great  body 
and  a  small  body  attract  each  other,  both 
will  move,  but  the  motion  of  the  small 
body  will  be  so  much  more  than  that  of  the 
great  one  that  the  latter  will  seem,  relatively, 
127 


128  XHe  Solar  System 

to  stand  fast  while  the  small  one  moves. 
Then,  if  the  small  body  had  originally  a 
motion  across  the  direction  in  which  the 
great  body  attracts  it,  the  result  of  the  com- 
bination will  be  to  cause  the  small  body  to 
revolve  in  an  orbit  (more  or  less  elliptical 
according  to  the  direction  and  velocity  of 
its  original  motion)  about  the  great  body. 
If  the  difference  of  mass  is  very  great,  the 
large  body  will  remain  virtually  immovable. 
This  is  the  case  ^th  the  sun  and  its  planets. 
The  sun  has  332,000  times  as  much  mass 
(or,  we  may  say,  is  332,000  times  as  heavy) 
as  the  earth.  It  has  a  little  more  than  a 
thousand  times  as  much  mass  as  its  largest 
planet,  Jupiter.  It  has  millions  of  times  as 
much  as  the  greatest  comet.  The  conse- 
quence is  that  all  of  these  bodies  revolve 
around  the  sun,  in  orbits  of  various  degrees 
of  eccentricity,  while  the  sun  itself  remains 
practically  immovable,  or  just  swaying  a 
little  this  way  and  that,  like  a  huntsman 
holding  his  dogs  in  leash. 

The  distance  of  the  sun  from  the  earth- 
about  93,000,000  miles — has  been  determined 
by  methods  which  will  be  briefly  explained 
in  the  next  section.  Knowing  its  distance, 
it  is  easy  to  calculate  its  size,  since  the 


The  S\in  129 

apparent  diameter  of  all  objects  varies 
directly  with  their  distance.  The  diameter 
of  the  sun  is  thus  found  to  be  about  866,400 
miles,  or  nearly  no  times  that  of  the  earth. 
In  bulk  it  exceeds  the  earth  about  1,300,000 
times,  but  its  mass,  or  quantity  of  matter, 
is  only  332,000  times  the  earth's,  because 
its  average  density  is  but  one  quarter  that  of 
the  earth.  This  arises  from  the  fact  that 
the  earth  is  a  solid,  compact  body,  while  the 
sun  is  a  body  composed  of  gases  and  vapours 
(though  in  a  very  compressed  state).  It 
is  the  high  temperature  of  the  sun  which 
maintains  it  in  this  state.  Its  temperature 
has  been  calculated  at  about  16,000°  Fahr- 
enheit, but  various  estimates  differ  rather 
widely.  At  any  rate,  it  is  so  hot  that  the 
most  refractory  substances  known  to  us 
would  be  reduced  to  the  state  of  vapour, 
if  removed  to  the  sun.  The  quantity  of  heat 
received  upon  the  earth  from  the  sun  can 
only  be  expressed  in  terms  of  the  mechanical 
equivalent  of  heat.  The  unit  of  heat  in 
engineering  is  the  calorie,  which  means  the 
amount  of  heat  required  to  raise  the  tem- 
perature of  one  kilogram  of  water  (2.2  pounds) 
one  degree  Centigrade  (i°.8  Fahrenheit). 
Now  observation  shows  that  the  sun  fur- 


130  THe  Solar  System 

nishes  30  of  these  calories  per  minute  upon 
every  square  metre  (about  1.2  square  yard) 
of  the  earth's  surface.  Perhaps  there  is 
no  better  illustration  of  what  this  means 
than  Prof.  Young's  statement,  that  "the 
heat  annually  received  on  each  square  foot 
of  the  earth's  surface,  if  employed  in  a  perfect 
heat  engine,  would  be  able  to  hoist  about  a 
hundred  tons  to  the  height  of  a  mile."  Or 
take  Prof.  Todd's  illustration  of  the  me- 
chanical power  of  the  sunbeams:  "If  we 
measure  off  a  space  five  feet  square,  the 
energy  of  the  sun's  rays  when  falling  verti- 
cally upon  it  is  equivalent  to  one  horse 
power."  Astronomers  ordinarily  reckon  the 
solar  constant  in  "small  calories,"  which  are 
but  the  thousandth  part  of  the  engineer's 
calorie,  and  the  latest  results  of  the  Smith- 
sonian Institution  observations  indicate  that 
the  solar  constant  is  about  1.95  of  these 
small  calories  per  square  centimeter  per 
second.  About  30  per  cent,  must  be  de- 
ducted for  atmospheric  absorption. 

Heat,  like  gravitation  and  like  light, 
varies  inversely  in  intensity  with  the  square 
of  the  distance;  hence,  if  the  earth  were 
twice  as  near  as  it  is  to  the  sun  it  would 
receive  four  times  as  much  heat  and  four 


The  Sun  131 

times  as  much  light,  and  if  it  were  twice 
as  far  away  it  would  receive  only  one  quarter 
as  much.  This  shows  how  important  it  is 
for  a  planet  not  to  be  too  near,  or  too  far 
from,  the  sun.  The  earth  would  be  vapour- 
ised  *if  it  were  carried  within  a  quarter  of 
a  million  miles  of  the  sun. 

The  sun  rotates  on  an  axis  inclined  about 
7J^°  from  a  perpendicular  to  the  plane 
of  the  ecliptic.  The  average  period  of  its 
rotation  is  about  25 J  days — we  say  "aver- 
age" because,  not  being  a  solid  body,  different 
parts  of  its  surface  turn  at  different  rates. 
It  rotates  faster  at  the  equator  than  at 
latitudes  north  and  south  of  the  equator,  the 
velocity  decreasing  toward  the  poles.  The 
period  of  rotation  at  the  equator  is  about 
25  days,  and  at  40°  north  or  south  of  the 
equator  it  is  about  27  days.  The  direction 
of  rotation  is  the  same  as  that  of  the  earth's. 

The  surface  of  the  sun,  when  viewed  with 
a  telescope,  is  often  seen  more  or  less  spotted. 
The  spots  are  black,  or  dusky,  and  frequently 
of  very  irregular  shapes,  although  many  of 
them  are  nearly  circular.  Generally  they 
appear  in  groups  drawn  out  in  the  direction 
of  the  solar  rotation.  Some  of  these  groups 
cover  areas  of  many  millions  of  square  miles, 


132  THe  Solar  System 

although  the  sun  is  so  immense  that  even 
then  they  appear  to  the  naked  eye  (guarded 
by  a  dark  glass)  only  as  small  dark  spots  on 
its  surface.  The  centres  of  sun-spots,  are  the 
darkest  parts.  Generally  around  the  borders 
of  the  spots  the  surface  seems  to  be  more  or 
less  heaped  up.  Often,  in  large  sun-spots, 
immense  promontories,  very  brilliant,  project 
over  the  dark  interior,  and  many  of  these  are 
prolonged  into  bridges  of  light,  apparently 
traversing  the  chasms  beneath.  Constant 
changes  of  shape  and  arrangement  take  place, 
and  there  are  few  more  astonishing  telescopic 
objects  than  a  great  sun-spot. 

The  spots  are  not  always  visible  in  equal 
numbers,  and  in  some  years  but  few  are  seen, 
and  they  are  small.  It  has  been  found  that 
they  occur  in  periods,  averaging  about  eleven 
years  from  maximum  to  minimum,  although 
the  length  of  the  period  is  very  irregular. 
It  has  also  been  observed  that  when  the  first 
spots  of  a  new  period  appear,  they  are 
generally  seen  some  30°  from  the  equator, 
either  toward  the  north  or  toward  the  south, 
and  that  as  the  period  progresses  the  spots 
increase  in  size,  and  seem  to  draw  toward  the 
equator,  the  last  spots  of  the  period  being 
seen  quite  close  to  the  equator,  on  one  side 


"  Tress  Nebula"  (N.,  G.  C.  6992)  in  Cygnus 

Photographed  at  the  Yerkes  Observatory  by  G.  W.Ritchey,  with  the  two- 
foot  reflector. 

Observe  the  strangely  twisted  look  of  this  long  curved  nebula  ;  also  the 
curious  curves  composed  of  minute  stars  near  it. 


The  Sun  133 

or  the  other.  The  duration  of  individual 
spots  is  variable;  some  last  but  a  day  or 
two,  and  others  continue  for  weeks,  some- 
times being  carried  out  of  view  by  the  rota- 
tion of  the  sun  and  brought  into  view  again 
from  the  other  side. 

The  surface  of  the  sun  in  the  neighbour- 
hood of  groups  of  spots  is  frequently  marked 
by  large  areas  covered  with  crinkled  bright 
lines  and  patches,  which  are  called  faculae. 
These,  which  are  the  brightest  parts  of  the 
sun,  appear  to  be  elevated  above  the  general 
level. 

As  to  the  cause  and  nature  of  sun-spots 
much  remains  to  be  learned.  In  1908,  Prof. 
George  E.  Hale,  by  means  of  an  instrument 
called  the  spectro-heliograph,  which  selects 
out  of  the  total  radiation  of  the  solar  disk 
light  peculiar  to  certain  elements,  and  thus 
permits  the  use  of  that  light  alone  in  photo- 
graphing the  sun,  demonstrated  that  sun- 
spots  probably  arise  from  vortices,  or 
whirling  storms,  and  that  these  vortices 
produce  strong  magnetic  fields  in  the  sun- 
spots.  The  phenomenon  may  be  regarded, 
says  Prof.  Hale,  as  somewhat  analogous  to 
a  tornado  or  waterspout  on  the  earth.  The 
whirling  trombe  becomes  wider  at  the  top, 


134  THe  Solar  System 

carrying  the  gases  from  below  upward.  At 
the  centre  of  the  storm  the  rapid  rotation 
produces  an  expansion  which  cools  the 
gases  and  causes  the  appearance  of  a  com- 
paratively dark  cloud,  which  we  see  as  the 
sun-spot.  The  vortices  whirl  in  opposite 
directions  on  opposite  sides  of  the  sun's 
equator,  thus  obeying  the  same  law  that 
governs  the  rotation  of  cyclones  on  the 
earth. 

It  has  long  been  a  question  whether  the 
condition  of  the  sun  as  manifested  by  the 
spots  upon  its  surface  has  an  influence  upon 
the  meteorology  of  the  earth.  It  is  known 
that  the  sun-spot  period  coincides  closely 
with  periodical  changes  in  the  earth's  mag- 
netism, and  great  outbursts  on  the  sun  have 
frequently  been  immediately  followed  by 
violent  magnetic  storms  and  brilliant  displays 
of  the  aurora  borealis  on  the  earth. 

The  sun  undoubtedly  exercises  other  in- 
fluences upon  the  earth  than  those  familiar 
to  us  under  the  names  of  gravitation,  light, 
and  heat;  but  the  nature  of  these  other 
influences  is  not  yet  fully  understood. 

The  brilliant  white  surface  of  the  sun  is 
called  the  photosphere.  It  has  been  likened 
to  a  shell  of  intensely  hot  clouds,  consisting 


TKe  Sxin  135 

of  substances  which  are  entirely  vaporous 
within  the  body  of  the  sun.  Above  the 
photosphere  lies  an  envelope,  estimated  to 
be  from  5000  to  10,000  miles  thick,  known 
as  the  chromosphere.  It  consists  mainly 
of  hydrogen  and  helium,  and  when  seen 
during  a  total  eclipse,  when  the  globe  of 
the  sun  is,  concealed  behind  the  moon,  it  pre- 
sents a  brilliant  scarlet  colour.  Above  this 
are  frequently  seen  splendid  red  flame-like 
objects,  named  prominences.  They  are  of 
two  varieties — one  cloud-like  in  appearance, 
and  the  other  resembling  spikes,  or  trees 
with  spreading  tops, — but  often  their  forms 
are  infinitely  varied.  The  latter,  the  so- 
called  eruptive  prominences,  exhibit  rapid 
motion  away  from  the  sun's  surface,  as  if 
they  consisted  of  matter  which  has  been 
ejected  by  explosion.  Occasionally  these 
objects  have  been  seen  to  grow  to  a  height 
of  several  hundred  thousand  miles,  with 
velocities  of  two  or  three  hundred  miles 
per  second. 

The  sun  has  still  another  envelope,  of 
changing  form, — the  corona.  This  appar- 
ently consists  of  rare  gaseous  matter,  whose 
characteristic  constituent  is  an  element  un- 
known on  the  earth,  called  coronium.  The 


136  THe  Solar  System 

corona  appears  in  the  form  of  a  luminous 
halo,  surrounding  the  hidden  sun  during 
a  total  eclipse,  and  it  often  extends  outward 
several  million  miles.  Its  shape  varies  in 
accordance  with  the  sun-spot  period.  It  has 
a  different  appearance  and  outline  at  a  time 
of  maximum  sun-spots  from  those  which  it 
presents  at  a  minimum.  There  are  many 
things  about  the  corona  which  suggest  the 
play  of  electric  and  magnetic  forces.  The 
corona,  although  evidently  always  existing, 
is  never  seen  except  during  the  few  minutes 
of  complete  obscuration  of  the  sun  that 
occurs  in  a  total  eclipse.  This  is  because 
its  light  is  not  sufficiently  intense  to  render 
it  visible,  when  the  atmosphere  around  the 
observer  is  illuminated  by  the  direct  rays 
of  sunlight. 

2.  Parallax.  We  now  return  to  the 
question  of  the  sun's  distance  from  the  earth, 
which  we  treat  in  a  separate  section,  because 
thus  it  is  possible  to  present,  at  a  single 
view,  the  entire  subject  of  the  measurement 
of  the  distances  of  the  heavenly  bodies. 
The  common  basis  of  all  such  measurements 
is  furnished  by  what  is  called  parallax,  which 
may  be  defined  as  the  difference  of  direction 
of  an  object  when  viewed  alternately  from 


Parallax  137 

two  separate  points.  The  simplest  example 
of  parallax  is  found  in  looking  at  an  object 
first  with  one  eye  and  then  with  the  other 
without,  in  the  meantime,  altering  the 
position  of  the  head.  Suppose  you  sit  in 
front  of  a  window  through  which  you  can 
see  the  wall  of  a  house  on  the  opposite  side 
of  the  street.  Choose  one  of  the  vertical 
bars  of  the  window-sash,  and,  closing  the 
left  eye,  look  at  the  bar  with  the  right  and 
note  where  it  seems  to  be  projected  against 
the  wall.  Then  close  the  right  eye  and  open 
the  left,  and  you  will  observe  that  the  place 
of  projection  of  the  bar  has  shifted  toward 
the  right.  This  change  of  direction  is  due 
to  parallax  and  its  amount  depends  both 
upon  the  distance  between  the  eyes  and  upon 
the  distance  of  the  window  from  the  observer. 
To  see  how  this  principle  is  applied  by  the 
astronomer,  let  us  suppose  that  we  wish  to 
ascertain  the  distance  of  the  moon.  The 
moon  is  so  far  away  that  the  distance  between 
the  eyes  is  infinitesimal  in  comparison, 
so  that  no  parallactic  shift  in  its  direction 
is  apparent  on  viewing  it  alternately  with 
the  two  eyes.  But  by  making  the  obser- 
vations from  widely  separated  points  on 
the  earth  we  can  produce  a  parallactic 


138  The  Solar  System 

shifting  of  the  moon's  position  which  will 
be  easily  measurable. 

Let  one  of  the  points  of  observation  be 
in  the  northern  hemisphere  and  the  other 
in  the  southern,  thousands  of  miles  apart. 
The  two  observers  might  then  be  compared 
to  the  eyes  of  an  enormous  head,  each  of 
which  sees  the  moon  in  a  measurably 
different  direction.  If  the  northern  observer 
carefully  ascertains  the  angular  distance 
of  the  moon  from  his  zenith,  and  the  southern 
observer  does  the  same  with  regard  to  his 
zenith,  as  indicated  in  Fig.  12,  they  can, 
by  a  combination  of  their  measurements, 
construct  a  quadrilateral  A  c  B  M,  of  which 
all  the  angles  may  be  ascertained  from  the 
two  measurements,  while  the  length  of  the 
sides  A  c  and  B  c  is  already  known,  since 
they  are  each  equal  to  the  radius  of  the 
earth.  With  these  data  it  is  easy,  by  the 
rules  of  plane  trigonometry,  to  calculate 
the  length  of  the  other  sides,  and  also  the 
length  of  the  straight  line  from  the  centre 
of  the  earth  to  the  moon.  In  all  such 
cases  the  distance  between  the  points  of 
observation  is  called  the  base-line,  whose 
length  is  known  to  start  with,  while  the 
angles  formed  by  the  lines  of  direction  at 


Parallax 


139 


the  opposite  ends  of  the  base-line  are  ascer- 
tained by  measurement. 

In  the  case  of  the  sun  the  distance  con- 


oo  n. 


Fig.  12.     Parallax  of  tk 

Let  C  be  the  centre  of  the  earth,  A  and  B  the  stations  of 
two  observers,  one  in  the  northern,  the  other  in  the 
southern  hemisphere,  and  M  the  moon.  The  lines  C  A  Z 
and  C  B  Z'  indicate  the  direction  of  the  zenith  at  A 
and  B  respectively.  Subtracting  the  measured  angles 
at  A  and  B  each  from  180°  gives  the  inside  angles  at 
those  points.  The  angle  at  C  is  equal  to  the  sum  of  the 
latitudes  of  A  and  B  since  they  are  on  opposite  sides  of 
the  equator.  With  three  angles  known,  the  fourth,  at 
M,  is  found  by  simply  subtracting  their  sum  from  360°. 

cerned  is  so  great  (about  400  times  that  of 
the  moon)  that  the  parallax  produced  by 
viewing  it  from  different  points  on  the  earth 
is  too  small  to  be  certainly  measured,  and 


140  THe  Solar  System 

a  modification  of  the  method  has  to  be 
employed.  One  such  modification,  which 
has  been  much  used,  depends  upon  the  fact 
that  the  planet  Venus,  being  nearer  the  sun 
than  the  earth  is,  appears,  at  certain  times, 
passing  directly  over  the  face  of  the  sun. 
This  is  called  a  transit  of  Venus.  During 
a  transit,  Venus  is  between  three  and  four 
times  nearer  the  earth  than  the  sun  is,  and 
consequently  its  parallactic  displacement, 
when  viewed  from  widely  separated  points 
on  the  earth,  is  much  greater  than  that  of 
the  sun.  One  of  the  ways  in  which  the 
astronomer  takes  advantage  of  this  fact  is 
shown  in  Fig.  13.  Let  A  and  B  be  two 
points  on  opposite  sides  of  the  earth,  but 
both  somewhere  near  the  equator.  As  Venus 
swings  along  in  its  orbit  to  pass  between 
the  earth  and  the  sun,  it  will  manifestly  be 
seen  just  touching  the  sun's  edge  sooner 
from  A  than  from  B.  The  observer  at  A 
notes  with  extreme  accuracy  the  exact 
moment  when  he  sees  Venus  apparently 
touch  the  sun.  Several  minutes  later,  the 
observer  at  B  will  see  the  same  phenomenon, 
and  he  also  notes  accurately  the  time  of  the 
apparent  contact.  Now,  since  we  know  from 
ordinary  observation  the  time  that  Venus 


The  Great  Andromeda  Nebula 

Photographed  at  the  Yerkes  Observatory  by  G.  W.  Ritchey,  with  the  two- 
foot  reflector. 

Observe  the  vast  spiral,  or  elliptic,  rings  surrounding  the  central  con- 
densation and  the  appearance  of  breaking  up  and  re-shaping  into  smaller 
masses  which  some  of  the  rings  present. 


Parallax 


141 


requires  to  make  one  complete  circuit  of 
its  orbit,  we  can,  by  simple  proportion, 
calculate,  from  the  time  that  it  takes  to  pass 
from  v  to  v1,  the  angular  distance  between 
the  lines  A  S  and  B  S,  or  in  other  words  the 
size  of  the  angle  at  S,  which  is  equal  to  the 
parallactic  displacement  of  the  sun,  as  seen 
from  opposite  ends  of  the  earth's  diameter. 
Knowing,  to  begin  with,  the  distance  between 


B 


Fig.  ij.     Parallax  of  the  Sun  from  Transit  of  Venus. 
(For  description  see  text.; 

A  and  B,  we  have  the  means  of  determining 
the  length  of  all  the  other  lines  in  the  triangle, 
and  hence  the  distance  of  the  sun.  This 
process  is  known  as  Delisle's  method. 
There  is  another  method,  called  Halley's, 
but  in  a  brief  treatise  of  this  kind  we  cannot 
enter  into  a  description  of  it.  It  suffices 
to  say  that  both  depend  upon  the  same  fun- 
damental principles. 

It  must   be   added,  however,  that  other 
ways  of  measuring  the  sun's  distance  than 


142  TKe  Solar  System 

are  afforded  by  transits  of  Venus  have  been 
developed.  One  of  these  depends  upon 
observation  of  the  asteroid  Eros,  which 
periodically  approaches  much  nearer  to  the 
earth  than  Venus  ever  does.  By  observing 
the  parallax  of  Eros,  when  it  is  nearest  the 
earth,  its  distance  can  be  ascertained,  and 
that  being  known  the  distance  of  the  sun 
is  immediately  deducible  from  it,  because, 
by  the  third  law  of  Kepler  (to  be  explained 
later),  the  relative  distances  of  all  the  planets 
from  the  sun  are  proportional  to  their 
periods  of  revolution,  so  that  if  we  know  any 
one  of  the  distances  in  miles  we  can  calcu- 
late all  the  others.  It  is  important  here  to 
state  the  angular  amount  of  the  sun's  parallax, 
since  it  is  a  quantity  which  is  continually 
referred  to  in  books  on  astronomy.  Accord- 
ing to  the  latest  determination,  based  on 
observations  of  Eros,  the  solar  parallax  is 
8 ".807,  which  corresponds,  in  round  num- 
bers, to  a  distance  of  92,800,000  miles.  A 
mean  parallax  of  8".796  is  given  by  Mr.  C.  G. 
Abbot,  based  on  a  combination  of  results 
from  a  number  of  different  methods,  and  this 
corresponds  to  a  distance  of  92,930,000  miles. 
To  the  astronomer,  who  seeks  extreme  ex- 
actness, the  slightest  difference  is  important  * 


Parallax  143 

It  should  be  noted  that  the  figures  8". 807  or 
8 ".796  represent  the  parallactic  displacement 
of  the  sun,  as  seen  not  from  the  opposite 
ends  of  the  earth's  entire  diameter,  but  from 
opposite  ends  of  its  radius,  or  semi-diameter. 
Accordingly  it  is  equal  to  half  of  the  angle 
at  S  in  Fig.  13.  It  is  for  convenience  of 
calculation  that,  in  such  cases,  the  astronomer 
employs  the  semi-diameter,  instead  of  the 
whole  diameter  for  his  base-line. 

The  case  of  the  stars  must  next  be  con- 
sidered, and  now  we  find  that  the  distances 
involved  are  so  enormous  that  the  diameter, 
or  semi-diameter,  of  the  earth  is  altogether 
too  insignificant  a  quantity  to  afford  an 
available  base-line  for  the  measurement. 
We  should  have  remained  forever  ignorant 
of  star  distances  but  for  the  effects  produced 
by  the  earth's  change  of  place  due  to  its 
annual  revolution  round  the  sun.  The  mean 
diameter  of  the  earth's  orbit  is  about 
186,000,000  miles,  and  we  are  able  to  make 
use  of  this  immense  distance  as  a  base-line 
for  ascertaining  the  parallax  of  a  star. 
Suppose,  for  instance,  that  the  direction 
of  a  star  in  the  sky  is  observed  on  the  1st 
of  January,  and  again  on  the  1st  of  July. 
In  the  meantime,  the  earth  will  have  passed 


144  THe  Solar  System 

from  one  end  of  the  base-line  just  described 
to  the  other,  and  unless  the  star  observed 
is  extremely  remote,  a  careful  comparison 
of  the  two  measurements  of  direction  will 
reveal  a  perceptible  parallax,  from  which 
the  actual  distance  of  the  star  in  question 
can  be  deduced. 

It  is  to  be  observed  that  if  all  the  stars 
were  equally  distant  this  method  would  fail, 
because  then  there  would  be  no  "  background  " 
against  which  the  shift  of  place  could  be 
observed ;  all  of  the  stars  would  shift  together. 
But,  in  fact,  the  vast  majority  of  the  stars 
are  so  remote  that  even  a  base-line  of 
186,000,000  miles  is  insufficient  to  produce 
a  measurable  shift  in  their  direction.  It  is 
only  the  distances  of  the  nearer  stars  which 
we  can  measure,  and  for  them  the  multitude 
of  more  remote  ones  serves,  like  the  wall  of 
the  house  in  the  experiment  with  the  window- 
bar,  as  a  background  on  which  the  shift 
of  place  can  be  noted.  Just  as  in  calculations 
of  the  sun's  parallax  the  semi-diameter  of 
the  earth  is  chosen  for  a  base-line,  so  in  the 
case  of  the  stars  the  semi -diameter  of  the 
earth's  orbit,  amounting  to  93,000,000  miles, 
forms  the  basis.  Measured  in  this  way  the 
parallaxes  of  the  nearest  stars  come  out  in 


Spectroscopic  Analysis          145 

tenths,  or  hundredths,  of  a  second  of  arc, 
or  angular  measurement.  Thus  the  parallax 
of  Alpha  Centauri,  the  nearest  known  starf 
is  about  o".75,  corresponding  to  a  distance 
of  about  26,000,000,000,000  miles.  Now 
o".75  is  a  quantity  inappreciable  to  the  naked 
eye,  and  only  to  be  measured  with  delicate 
instruments,  and  yet  this  almost  invisible 
shift  of  direction  is  all  that  is  produced  by 
viewing  the  nearest  star  in  the  sky  from  the 
opposite  ends  of  a  base-line  93,000,000  miles 
long! 

3.  Spectroscopic  Analysis.  We  have  next 
to  deal  with  the  constitution  of  the  sun, 
or  the  nature  of  the  substances  of  which  it 
consists,  and  for  this  purpose  we  must  first 
understand  the  operation  of  the  spectroscope, 
in  many  respects  the  most  wonderful  in- 
strument that  man  has  invented.  It  has 
given  birth  to  the  "chemistry  of  the  sun" 
and  the  "chemistry  of  the  stars/'  for  by  its 
aid  we  can  be  as  certain  of  the  nature  of  many 
of  the  substances  of  which  they  are  made 
as  we  could  be  by  actually  visiting  them. 

Fundamentally,  Spectroscopic  analysis  de- 
pends upon  the  principle  of  refraction,  of 
which  we  have  spoken  in  connection  with 
the  atmosphere.  Although  the  most  power- 

10 


146  THe  Solar  System 

ful  spectroscopes  are  now  made  on  a  different 
plan,  the  working  of  the  instrument  can 
best  be  comprehended  by  considering  it  in 
the  form  in  which  it  was  first  invented,  and 
in  which  it  is  still  most  often  used.  In  its 
simplest  form  the  spectroscope  consists  of 
a  three-angle  prism  of  glass,  through  which  a 
ray  of  light  is  sent  from  the  sun,  star,  or 
other  luminous  object  to  be  examined. 
Glass,  like  air  or  water,  has  the  property 
of  refracting,  or  bending,  all  rays  of  light 
that  enter  it  in  an  inclined  direction.  In 
passing  through  two  of  the  opposite-sloping 
sides  of  a  prism,  the  ray  is  twice  bent,  once 
on  entering  and  again  on  leaving,  in  accord- 
ance with  the  principle  that  we  have  already 
mentioned  (see  Part  II,  Sect.  4).  Still, 
merely  bending  the  ray  out  of  its  original 
course  would  have  no  important  result 
but  for  another  associated  phenomenon, 
known  as  dispersion.  To  explain  dispersion 
we  must  recall  the  familiar  fact  that  white 
light  consists  of  a  number  of  coloured  com- 
ponents which,  when  united,  make  white. 
It  is  usual  to  speak  of  these  primary,  or 
prismatic,  colours,  as  seven  in  number. 
These  are  red,  orange,  yellow,  green,  blue, 
indigo,  and  violet.  Physicists  now  assign 


Spectroscopic  Analysis         147 


Fig.  14.     Spectrum  Analysis. 

The  red  is  least  turned  by  the  prism  from  its  original 
course  and  the  violet  most.  If  between  the  prism  and 
the  screen  on  which  the  spectrum  falls  there  were  inter- 
posed a  gas  of  any  kind  that  gas  would  absorb  from  the 
coloured  rays  passing  through  it  the  exact  waves  of  light 
with  which  it  would  itself  shine  if  it  were  made  luminous 
by  heat.  It  would  not  take  out  an  entire  section,  or 
colour,  from  the  spectrum,  but  only  a  small  part  of  one 
or  more  of  the  colours,  and  the  absence  of  these  parts 
would  be  indicated  on  the  screen  by  narrow  black  lines 
situated  in  various  places ;  and  these  lines,  in  number  and 
in  situation,  would  differ  with  every  different  kind  of 
gas  that  was  interposed.  If  several  kinds  were  inter- 
posed simultaneously  they  would  all  pick  out  their  own 
peculiar  rays  from  the  light,  and  thus  the  spectrum 
would  be  crossed  by  a  large  number  of  dark  lines,  by  the 
aid  of  which  the  nature  of  the  various  gases  that  pro- 
duced them  could  be  told.  The  effect  would  be  the 
same  if  the  gases  were  interposed  in  the  path  of  the 
white  light  before  it  enters  the  prism; — and  this,  in  fact, 
is  what  happens  when  the  spectrum  of  the  sun,  or  a  star, 
is  examined — the  absorption  has  already  occurred  at  the 
surface  of  the  luminous  body  before  the  light  comes  to 
the  earth. 


148  THe  Solar  System 

a  different  list  of  primary  colours,  but  these, 
being  generally  familiar,  will  best  serve  our 
purpose.  Without  going  into  an  explanation 
of  the  reasons,  it  will  suffice  to  say  that  the 
waves  of  light  producing  these  fundamental 
colours  are  not  all  equally  affected  by  refrac- 
tion. The  red  is  least,  and  the  violet  most, 
bent  out  of  its  course  in  passing  through  the 
prism,  the  other  colours  being  bent  more  and 
more  in  proportion  to  their  distance  from  the 
red.  It  follows  that  the  ray,  or  beam,  of 
light,  which  was  white  when  it  entered  the 
prism,  becomes  divided  or  dispersed  during 
its  passage  into  a  brush  of  seven  different 
hues.  Thus  the  prism  may  be  said  to  ana- 
lyse the  light  into  its  fundamental  colours, 
making  them  separately  visible.  This,  as 
a  scientific  fact,  dates  from  the  time  of  New- 
ton. But  Newton  did  not  dream  of  the 
further  magic  that  lay  in  the  prism. 

It  was  noticed  as  early  as  1801  that,  when 
the  light  of  the  sun  was  dispersed  in  the  way 
we  have  described,  not  only  did  the  seven 
primary  colours  make  their  appearance, 
but  across  the  ribbon-like  band,  called  the 
spectrum,  that  was  thus  formed,  ran  a 
number  of  thin  black  lines,  like  narrow  gaps 
in  the  band.  The  position  of  these  lines  was 


Spectroscopic  Analysis         149 

carefully  studied  by  a  German  astronomer, 
Fraunhofer,  in  1814,  and  they  still  bear  the 
name  of  Fraunhofer  lines.  But  the  full 
explanation  of  them  did  not  come  until  1858 
when,  with  their  aid,  Kirschoff  laid  the  founda- 
tions of  spectrum  analysis. 

This  analysis  is  based  upon  the  fact  that 
the  Fraunhofer  lines  are  visual  indications  of 
the  existence  of  certain  substances  in  the 
sun.  To  explain  this  we  must  know  three 
fundamental  facts : 

ist :  Every  incandescent  body  that  is  either 
solid  or  liquid  (or,  if  it  consists  of  gases, 
is  under  high  pressure)  shines  with  compound 
white  light,  which,  when  dispersed  by  prisms, 
gives  a  continuous  coloured  band,  or  spectrum. 

2d:  Every  elementary  substance  when  in 
the  gaseous  state,  and  under  low  pressure, 
if  brought  to  incandescence  by  heat,  shines 
with  light  which,  when  dispersed,  gives  a 
discontinuous  spectrum,  made  up  of  separate 
bright  lines;  and  each  different  element 
possesses  its  own  peculiar  spectral  lines, 
never  coinciding  in  position  with  the  lines 
of  any  other  element. 

3d:  If  the  light  from  a  body  giving  a 
continuous  spectrum  is  caused  to  pass 
through  a  gas  which  is  at  a  lower  temperature, 


ISO  The  Solar  System 

the  gas  will  absorb  precisely  those  light 
waves,  of  which  its  own  spectrum  is  composed 
and  will  leave  in  the  spectrum  of  the  body 
a  series  of  dark  lines,  or  gaps,  whose  number 
and  position  indicate  the  nature  of  the  gas 
whose  absorptive  action  has  produced  them. 

Now,  to  apply  these  principles  to  the  sun 
we  have  only  to  remember  that  it  is  a  globe 
of  gaseous  substances,  which  are  under 
great  pressure,  owing  to  the  immense  force 
of  the  sun's  gravitation.  Consequently  it 
gives  a  continuous  spectrum.  But,  at  the 
same  time,  it  is  surrounded  with  gaseous 
envelopes,  which  are  not  as  much  com- 
pressed as  the  internal  gases  are,  and  which 
are  at  a  lower  temperature  because  they 
come  in  contact  with  the  cold  of  surround- 
ing space.  The  light  from  the  body  of  the 
sun  must  necessarily  pass  through  these 
envelopes,  and  each  of  the  gases  of  which 
they  consist  absorbs  from  the  passing  sun- 
light its  own  peculiar  rays,  with  the  result 
that  the  spectrum  of  the  sun  is  seen  crossed 
with  a  great  number  of  black  lines — the 
Fraunhofer  lines. 

It  will  be  remarked  that  the  evidence 
which  the  Fraunhofer  lines  afford  concerning 
the  composition  of  the  sun  applies,  strictly t 


Spectroscopic  Analysis          151 

only  to  the  outer  portion,  or  to  the  envelopes 
of  gaseous  matter  that  surround  the  interior 
globe.  But  since  there  is  every  reason  to 
believe  that  the  entire  body  of  the  sun  is 
in  &  gaseous  state,  notwithstanding  the 
internal  pressure,  and  since  we  see  that 
there  is  a  continual  circulation  going  on 
between  the  inner  and  outer  portions,  it  is 
logical  to  conclude  that  essentially  the  same 
elements  exist  under  varying  conditions  in 
all  parts  of  the  sun. 

In  this  way,  then,  we  have  learned  the 
composition  of  the  sun,  and  we  find  that  it 
consists  of  virtually  the  same  elementary 
substances  found  upon  the  earth,  but  exist- 
ing there  in  a  gaseous  or  vaporous  state. 
Among  the  elements  which  have  been 
positively  identified  in  the  sun  by  means  of 
their  characteristic  spectral  lines  are  iron, 
calcium,  sodium,  aluminum,  copper,  zinc, 
silver,  lead,  potassium,  nickel,  tin,  silicon, 
manganese,  magnesium,  cobalt,  hydrogen, 
and  at  least  twenty  others  which  are  likewise 
found  upon  the  earth.  Some  elementary 
substances  known  on  the  earth,  such  as 
gold  and  oxygen,  have  not  yet  been  certainly 
found  in  the  sun,  but  there  is  every  reason 
to  believe  that  they  all  exist  there,  though 


152  TKe  Solar  System 

perhaps  under  conditions  which  render  their 
detection  difficult  or  impossible.  Helium 
was  recognised  as  an  element  in  the  sun, 
by  giving  spectral  lines  different  from  any 
known  substance,  and  it  received  its  name 
"sun-metal,"  long  before  it  was  discovered 
on  the  earth.  We  have  seen  that  there  is 
at  least  one  element  in  the  sun,  coronium, 
which,  as  far  as  we  know,  does  not  exist 
at  all  upon  the  earth,  and  it  is  not  improbable 
that  there  may  be  others  which  have  no 
counterparts  on  the  earth. 

The  same  kind  of  analysis  applies  to  the 
stars,  no  matter  how  far  away  they  may  be, 
so  long  as  they  give  sufficient  light  to  form 
a  spectrum.  And  in  this  way  it  has  been 
found  that  the  stars  differ  somewhat  from 
the  sun  and  from  one  another  in  their  com- 
position, and  thus  a  classification  of  the  stars 
has  been  made,  and  it  has  been  possible  to 
draw  conclusions  concerning  their  relative 
age,  which  show  that  some  stars  are  com- 
paratively younger  than  the  sun,  others 
older,  and  others  so  far  advanced  in  age, 
or  evolution,  that  they  are  drawing  near 
extinction.  Many  dark  bodies  also  exist 
among  the  stars,  which  appear  to  be  com- 
pletely extinguished  suns.  It  only  remains 


THe  Moon  153 

to  add  on  this  subject  that,  according  to 
prevailing  theories,  the  earth  itself  was  once 
an  incandescent  body,  shining  with  its  own 
light,  and  at  that  time  it,  too,  would  have 
yielded  a  spectrum  showing  of  what  sub- 
stances it  consisted. 

4.  Thfe  Moon.  The  earth  is  a  satellite 
of  the  sun,  and  the  moon  is  a  satellite  of  the 
earth.  The  mean,  or  average,  distance  of 
the  sun  from  the  earth  is  about  93,000,000 
miles;  the  mean  distance  of  the  moon  is  a 
little  less  than  239,000  miles.  This  distance 
is  variable  to  the  extent  of  about  31,000 
miles,  owing  to  the  eccentricity  of  the  moon's 
orbit  about  the  earth.  That  is,  the  moon  is 
sometimes  nearly  253,000  miles  away,  and 
sometimes  only  about  22 1 ,600.  The  diameter 
of  the  moon,  is  2163  miles.  Its  bulk  is  one- 
forty-ninth  that  of  the  earth,  but  its  mass  is 
only  one-eightieth,  because  its  mean  density 
is  only  about  six- tenths  as  great  as  the  earth's. 

The  moon  appears  to  travel  in  an  orbit 
round  the  earth,  but  in  fact  the  orbit  is 
always  concave  toward  the  sun,  and  the 
disturbing  attraction  of  the  earth,  as  the 
two  move  together  round  the  sun,  causes 
it  to  appear  now  on  one  side  and  now  on  the 
other.  But  we  may  treat  the  moon's 


154  TKe  Solar  System 

orbit  as  if  the  earth  were  the  true  centre 
of  force,  the  attraction  of  the  sun  being  re- 
garded as  the  disturbing  element. 

According  to  a  mathematical  theory,  which 
has  been  largely  accepted  as  probably  true, 
but  into  which  we  cannot  enter  here  (see 
Prof.  George  Darwin's  The  Tides,  or  Prof. 
R.  Ball's  Time  and  Tide),  the  moon  was 
thrown  off  from  the  earth  many  ages  ago, 
as  a  consequence  of  tidal  "friction."  As  it 
moves  round  in  its  orbit  the  moon  keeps 
the  same  face  toward  the  earth.  This  fact  is 
also  ascribed  to  tidal  influence. 

Apparently  the  moon  has  no  atmosphere, 
or  if  it  has  any  it  is  too  rare  to  be  certainly 
detected.  On  its  surface,  there  is  no  appear- 
ance of  water.  Consequently  we  cannot 
suppose  it  to  be  inhabited,  at  least  by  any 
forms  of  life  familiar  to  us  on  the  earth. 
But  when  the  moon  is  viewed  with  a  telescope 
large  relatively  flat  areas  are  seen,  which 
some  think  may  have  been  the  beds  of  seas 
in  ancient  times.  They  are  still  called  maria, 
or  "seas,"  and  are  visible  to  the  naked  eye 
in  the  form  of  great  irregular  dusky  regions. 
Nearly  two-thirds  of  the  surface  of  the  moon, 
as  we  see  it,  consists  of  bright  regions,  which 
are  very  broken  and  mountainous.  Most 


Spiral  Nebula  in  Cepheus     (H  IV  76) 

Photographed  at  the  Lick  Observatory  by  J.  E.  Keeler,  with  the  Crossley 

reflector.      Exposure  four  hours. 
Observe  that  the  central  portion  is  only  of  stellar  magnitude. 


Nebulous  Groundwork  in  Taurus 

Photographed  at  the   Yerkes  Observatory  by  E.  E.  Barnard   with  lo-inch 

Bruce  telescope.     Exposure   six  hours  twenty-eight  minutes. 
Prof.  Barnard  has  suggested  that  some  of  these  dark  lanes  in  rich  regions 
nf  stars  are  non-luminous  nebulae. 


The  Moon  155 

of  the  mountains  of  the  moon  are  roughly 
circular,  surrounding  enormous  depressions, 
which  look  like  gigantic  pits.  For  this 
reason  they  are  called  lunar  volcanoes,  but, 
to  say  nothing  of  their  immense  size — for 
many  are  fifty  or  sixty  miles  across — they 
differ  in  many  ways  from  the  volcanoes  of 
the  earth.  It  suffices  to  point  out  that  what 
resemble  volcanic  craters  are  not  situated, 
as  is  the  case  on  the  earth,  at  the  summits 
of  mountains,  but  are  vast  sink-holes,  de- 
scending thousands  of  feet  below  the  general 
surface  of  the  moon.  Their  real  origin  is 
unknown,  but  it  is  possible  that  volcanic 
forces  may  have  produced  them.  (For  a 
description,  with  photographs,  of  these 
gigantic  formations  in  the  lunar  world,  see 
the  present  author's  The  Moon.)  In  ad- 
dition to  the  circular  mountains,  or  craters, 
there  are  several  long  and  lofty  chains  of 
lunar  mountains  much  resembling  terrestrial 
mountain  ranges. 

As  to  the  absence  of  air  and  water  from 
the  moon,  some  have  supposed  that  they 
once  existed,  but,  in  the  course  of  ages, 
have  disappeared,  either  by  absorption, 
partly  mechanical  and  partly  chemical,  into 
the  interior  rocks,  or  by  escaping  into  space 


156  XHe  Solar  System 

on  account  of  the  slight  force  of  gravity  on 
the  moon,  which  appears  to  be  insufficient  to 
enable  it  to  retain,  permanently,  such  volatile 
gases  as  oxygen,  hydrogen,  and  nitrogen. 
This  leads  us  to  consider  the  force  of  the 
moon's  attraction  at  its  surface.  We  have 
seen  that  spherical  bodies  attract  as  if  their 
whole  mass  were  collected  at  their  centres. 
We  also  know  that  the  force  of  attraction 
varies  directly  as  the  mass  of  the  attracting 
body  and  inversely  as  the  square  of  the 
distance  from  its  centre.  Now  the  mass  of 
the  moon  is  one-eightieth  that  of  the  earth,  so 
that,  upon  bodies  situated  at  an  equal  dis- 
tance from  the  centres  of  both,  the  moon's 
attraction  would  be  only  one-eightieth  of 
the  earth's.  But  the  diameter  of  the  moon 
is  not  very  much  more  than  one  quarter 
that  of  the  earth,  and  for  the  sake  of  round 
numbers  let  us  call  it  one  quarter.  It 
follows  that  an  object  on  the  surface  of  the 
moon  is  four  times  nearer  the  centre  of 
attraction  than  is  an  object  on  the  surface 
of  the  earth,  and  since  the  force  varies 
inversely  as  the  square  of  the  distance  the 
moon's  attraction  upon  bodies  on  its  surface 
is  relatively  sixteen  times  as  great  as  the 
earth's.  But  the  total  force  of  the  earth's 


The  Moon  157 

attraction  is  eighty  times  greater  than  the 
moon's.  In  order,  then,  to  find  the  real 
relative  attraction  of  the  moon  at  its  surface 
we  must  divide  80  by  1 6,  the  quotient,  5, 
showing  the  ratio  of  the  earth's  force  of 
attraction  at  its  surface  to  that  of  the  moon 
at  its  surface.  In  other  words,  this  calcu- 
lation shows  that  the  moon  draws  bodies 
on  its  surface  with  only  one-fifth  the  force 
with  which  the  same  bodies  would  be  drawn 
on  the  earth's  surface.  The  weight  of  bodies 
of  equal  mass  would,  therefore,  be  only 
one-fifth  as  great  on  the  moon  as  on  the 
earth. 

But  the  real  difference  is  greater  than  this, 
for  we  have  used  round  numbers,  which 
exaggerated  the  size  of  the  earth  as  compared 
with  that  of  the  moon.  If  we  employ  the 
fractional  numbers  which  show  the  actual 
ratio  of  the  moon's  radius  (half-diameter)  to 
that  of  the  earth,  we  shall  find  that  the 
weight  of  the  same  body  would  be  only 
about  one-sixth  as  great  on  the  moon  as 
on  the  earth.  It  has  been  thought  that  this 
relative  lack  of  weight  on  the  moon  may 
account  for  the  gigantic  proportions  assumed 
by  its  craters,  since  the  same  ejective  force 
would  throw  volcanic  matter  to  a  much 


158  TKe  Solar  System 

greater  height  and  distance  there  than  on 
our  planet. 

The  connection  of  the  slight  force  of  gravity 
on  the  moon  with  its  ability  to  retain  an 
atmosphere  is  shown  by  the  following  con- 
siderations. It  is  possible  to  calculate  for 
any  planet  of  known  mass  the  velocity  with 
which  a  particle  would  have  to  move  in 
order  to  escape  from  the  control  of  that 
planet.  In  the  case  of  the  earth  this  critical 
velocity,  as  it  is  called,  amounts  to  about 
7  miles  per  second,  and  in  the  case  of  the 
moon  to  only  iJ/2  miles  per  second.  Now 
the  kinetic  theory  of  gases  informs  us  that 
their  molecules  are  continually  flying  in 
all  directions  with  velocities  varying  with 
the  nature  and  the  temperature  of  the  gas. 
The  maximum  velocity  of  the  molecules  of 
oxygen  is  1.8  miles  per  second,  of  hydrogen 
7.4  miles,  of  nitrogen  2  miles,  of  water 
vapour  2.5  miles.  It  is  evident,  then,  that 
the  force  of  the  earth's  attraction  is  sufficient 
permanently  to  retain  all  these  gases  except 
hydrogen,  and  in  fact  there  is  no  gaseous 
hydrogen  in  the  atmo  >here,  that  element 
being  found  on  the  earth  only  in  combination 
with  other  substances.  But  oxygen  and 
nitrogen,  which  constitute  the  bulk  of  the 


The  Moon  159 

atmosphere,  have  maximum  molecular  ve- 
locities much  less  than  the  critical  velocity 
above  described.  In  the  case  of  the  moon, 
however,  the  critical  velocity  is  less  than  those 
of  the  molecules  of  oxygen,  nitrogen,  and 
water  vapour,  to  say  nothing  of  hydrogen; 
therefore  the  moon  cannot  permanently 
retain  them.  We  say  " permanently,"  be- 
cause they  might  be  retained  for  a  time 
for  the  reason  that  the  molecules  of  a  gas 
fly  in  all  directions,  and  continual  collisions 
occur  among  them  in  the  interior  of  the 
gaseous  mass,  so  that  it  would  be  only  those 
at  the  exterior  of  the  atmosphere  which 
would  escape ;  but  gradually  all  that  remained 
free  from  combination  would  get  away. 

As  the  moon  travels  round  the  earth 
it  shows  itself  in  different  forms,  gradually 
changing  from  one  into  another,  which  are 
known  as  phases.  If  the  moon  shone  with 
light  of  its  own  its  outline  would  always  be 
circular,  like  the  sun's.  The  apparent  change 
of  form  is  due,  first,  to  its  being  an  opaque 
globe,  reflecting  the  sunlight  that  falls  upon 
it,  and  necessarily  Uuminated  on  only  one 
side  at  a  time;  and  second,  to  the  fact  that  as 
it  travels  round  the  earth  the  half  illuminated 
by  the  sun  is  sometimes  turned  directly 


160  THe  Solar  System 

toward  us,  at  other  times  only  partly  toward 
us,  and  at  still  other  times  directly  away 


CAST  QUARTER  *% 


NCWMOON  (    EARTH    I  FULL    MOON\ 


•-,- 


Fig.  15.     The  Phases  of  the  Moon. 

As  the  moon  goes  round  the  earth  in  the  direction  indi- 
cated by  the  arrows,  the  sun  remaining  always  on  the 
left-hand  side,  it  is  evident  that  the  illuminated  half  of 
the  moon  will  be  turned  away  from  the  earth  at  new 
moon,  and  toward  it  at  full  moon,  while  between  these 
positions  more  or  less  will  be  seen  according  to  the 
direction  of  the  moon  with  regard  to  the  sun. 

from  us.     When  it  is  in  that  part  of  its  orbit 
which  passes  between  the  sun  and  the  earth, 


THe  Moon  161 

the  moon,  so  to  speak,  has  its  back  turned 
to  us,  the  illuminated  side  being,  of  course, 
toward  the  sun.  It  is  then  invisible,  and 
this  unseen  phase  is  the  true  "new  moon." 
It  is  customary,  however,  to  give  the  name 
new  moon  to  the  narrow,  sickle-shaped 
figure,  which  it  shows  in  the  west,  after 
sunset,  a  few  days  after  the  date  of  the  real 
new  moon.  The  sickle  gradually  enlarges 
into  a  half  circle  as  the  moon  passes  away 
from  the  sun,  and  the  half  circle  phase, 
which  occurs  when  the  moon  arrives  at  a 
position  in  the  sky  at  right  angles  to  the 
direction  of  the  sun,  is  called  first  quarter. 
After  first  quarter  the  moon  begins  to  move 
round  behind  the  earth,  with  respect  to  the 
sun,  and  when  it  has  arrived  just  behind 
the  earth,  its  whole  illuminated  face  is 
turned  toward  the  earth,  because  the  sun, 
which  causes  the  illumination,  is  on  that 
same  side.  This  phase  is  called  full 
moon.  Afterward  the  moon  returns  round 
the  other  part  of  its  orbit  toward  its  origi- 
nal position  between  the  earth  and  the 
sun,  and  as  it  does  so,  it  again  assumes, 
first,  the  form  of  a  half  circle,  which  in  this 
case  is  called  third,  or  last,  quarter,  then 
that  of  a  sickle,  known  as  "old  moon,"  and 


162  TTKe  Solar  System 

finally  disappears  once  more  to  become  new 
moon  again. 

A  perfectly  evident  explanation  of  these 
changes  of  form,  clearer  than  any  description, 
can  be  graphically  obtained  in  this  way. 
Take  a  billiard  ball,  a  croquet  ball,  or  a 
perfectly  round,  smooth,  and  tightly  rolled 
ball  of  white  yarn,  and,  placing  yourself 
not  too  near  a  brightly  burning  lamp  and 
sitting  on  a  piano  stool  (in  order  to  turn 
more  easily),  hold  the  ball  up  in  the  light, 
and  cause  it  to  revolve  round  you  by  turning 
upon  the  stool.  As  it  passes  from  a  position 
between  you  and  the  lamp  to  one  on  the  oppo- 
site side  from  the  lamp,  and  so  on  round 
to  its  original  position,  you  will  see  its 
illuminated  half  go  through  all  the  changes 
of  form  exhibited  by  the  moon,  and  you  will 
need  no  further  explanation  of  the  lunar 
phases. 

The  Harvest  Moon  and  the  Hunter's 
Moon,  which  are  popularly  celebrated  not 
only  on  account  of  their  romantic  associa- 
tions, but  also  because  in  some  parts  of  the 
world  they  afford  a  useful  prolongation  of 
light  after  sunset,  occur  only  near  the  time 
of  the  autumnal  equinox,  and  they  are 
always  full  moons.  The  full  moon  nearest 


Nebula  in  Sagittarius     (M  8) 

Photographed  at  the  Lick  Observatory  by  J.   E.  Keeler,  with  the  Crossley 

reflector.      Exposure  three  hours. 

Note  the  clustering  of  stars  over  the  whole  field,  the  intricate  forms  of 
the  nebula,  and  particularly  the  curious  black  spots,  or"  holes,"  resembling 
drops  of  ink. 


Eclipses  163 

the  date  of  the  equinox,  September  23d,  is 
the  Harvest  Moon,  and  the  full  moon  next 
following  is  .  the  Hunter's  Moon.  Their 
peculiarity  is  that  they  rise,  for  several 
successive  evenings,  almost  at  the  same  hour, 
immediately  after  sunset.  This  is  due  to  the 
fact  that  at  that  time  of  the  year  the  ecliptic, 
from  which  the  moon's  path  does  not  very 
widely  depart,  is,  in  high  latitudes,  nearly 
parallel  with  the  horizon. 

The  full  moon  in  winter  runs  higher  in  the 
sky,  and  consequently  gives  a  brighter  light, 
than  in  summer.  The  reason  is  because, 
since  the  full  moon  must  always  be  opposite 
to  the  sun,  and  since  the  sun  in  winter  runs 
low,  being  south  of  the  equator,  the  full  moon 
rides  proportionally  high. 

5.  Eclipses.  We  have  mentioned  the 
connection  of  the  moon  with  the  tides,  but 
there  is  another  phenomenon  in  which  the 
moon  plays  the  most  conspicuous  part — 
that  of  eclipses.  There  are  two  kinds  of 
eclipses — solar  and  lunar.  In  the  former 
it  is  the  moon  that  causes  the  eclipse,  by 
hiding  the  sun  from  view;  and  in  the  latter 
it  is  the  moon  that  suffers  the  eclipse,  by 
passing  through  the  shadow  which  the  earth 
casts  into  space  on  the  side  away  from  the 


164  THe  Solar  System 

sun.  In  both  cases,  in  order  that  there  may 
be  an  eclipse  it  is  necessary  that  the  three 
bodies,  the  moon,  the  sun,  and  the  earth, 
shall  be  nearly  on  a  straight  line,  drawn 
through  their  centres.  Since  the  moon 
occupies  about  a  month  in  going  round  the 
earth  there  would  be  two  eclipses  in  every 
such  period  (one  of  the  sun  and  the  other 
of  the  moon),  if  the  moon's  orbit  lay  exactly 
in  the  plane  of  the  ecliptic,  or  of  the  earth's 
orbit.  But,  in  fact,  the  orbit  of  the  moon  is 
inclined  to  that  plane  at  an  angle  of  some- 
thing over  5°.  Even  so,  there  would  be 
eclipses  every  month  if  the  two  opposite 
points,  called  nodes,  where  the  moon  crosses 
the  plane  of  the  ecliptic,  always  lay  in  a 
direct  line  with  the  earth  and  the  sun;  but 
they  do  not  lie  thus.  If,  then,  the  moon 
comes  between  the  earth  and  the  sun  when 
she  is  in  a  part  of  her  orbit  several  degrees 
above  or  below  the  plane  of  the  ecliptic,  it 
is  evident  that  she  will  pass  either  above  or 
below  the  straight  line  joining  the  centres  of 
the  earth  and  the  sun,  and  consequently 
cannot  hide  the  latter.  But,  since  eclipses 
do  occur  in  some  months  and  do  not  occur 
in  others,  we  must  conclude  that  the  situa- 
tion of  the  nodes  changes,  and  such  is  the 


E-clipses  165 

fact.  In  consequence  of  the  conflicting 
attractions  of  the  sun  and  the  earth,  the 
orbit  of  the  moon,  although,  like  that  of 
the  earth,  it  always  retains  nearly  the  same  Vx 
shape  and  the  same  inclination,  swings  round  /x 
in  space,  so  that  the  nodes,  or  crossing 
points  on  the  ecliptic,  continually  change 
their  position,  revolving  round  the  earth. 
This  motion  may  be  compared  to  that  of 
the  precession  of  the  equinoxes,  but  it  is 
much  more  rapid,  a  complete  revolution 
occurring  in  a  period  of  about  nineteen  years. 
From  this  it  follows  that  sometimes  the 
moon  in  passing  its  nodes  will  be  in  a  line 
with  the  sun,  and  sometimes  will  not.  But, 
owing  to  the  fact  that  the  sun  and  moon  are 
not  mere  points,  but  on  the  contrary  present 
to  our  view  circular  disks,  each  about  half 
a  degree  in  diameter,  an  eclipse  may  occur 
even  if  the  moon  is  not  in  an  exact  line  with 
the  centres  of  the  sun  and  the  earth.  The 
edge  of  the  moon  will  overlap  the  sun,  and 
there  will  be  a  partial  eclipse,  if  the  centres 
of  the  two  bodies  are  within  one  degree 
apart.  Now,  the  inclination  of  the  moon's 
orbit  to  the  ecliptic  being  only  a  little  over 
5°,  it  is  apparent  that  in  approaching  one  of 
its  nodes,  along  so  gentle  a  slope,  it  will 


i66  THe  Solar  System 

come  within  less  than  a  degree  of  the  ecliptic 
while  still  quite  far  from  the  node.  Thus, 
eclipses  occur  for  a  considerable  time  before 
and  after  the  moon  passes  a  node.  The 
distances  on  each  side  of  the  node,  within 
which  an  eclipse  of  the  sun  may  occur,  are 
called  the  solar  ecliptic  limits,  and  they 
amount  to  18°  in  either  direction,  or  36°  in 
sum.  Within  these  limits  the  sun  may  be 
wholly  or  partially  eclipsed  according  as  the 
moon  is  nearer  to,  or  farther  from,  the  node. 
If  she  is  exactly  at,  or  very  close  to,  the  node 
the  eclipse  will  be  total. 

Solar  eclipses  vary  in  another  way.  What 
would  be  a  total  eclipse,  under  other  cir- 
cumstances, may  be  only  an  annular  eclipse 
if  the  moon  happens  to  be  near  her  greatest 
distance  from  the  earth.  We  have  described 
the  variations  in  her  distance  due  to  the 
eccentricity  of  her  orbit,  and  we  have  said 
that  the  orbit  itself  swings  round  the  earth 
in  such  a  way  as  to  cause  the  nodes  continu- 
ally to  change  their  places  on  the  ecliptic. 
The  motion  of  the  orbit  also  causes  the  lunar 
apsides,  or  the  points  where  she  is  at  her 
greatest  and  least  distances  from  the  earth, 
to  change  their  places,  but  their  revolution 
is  opposite  in  direction  to  that  of  the  nodes, 


E-clipses  167 

as  the  revolution  of  the  apsides  of  the  earth's 
orbit  is  opposite  to  that  of  the  equinoxes. 
The  moon's  apsides  sometimes  move  east- 
ward and  sometimes  westward,  but  upon  the 
whole  the  eastward  motion  prevails  and  the 
apsides  complete  one  revolution  in  that  direc- 
tion once  in  about  nine  and  one-half  years. 
In  consequence  of  the  combined  effects 
of  the  revolution  of  the  nodes  and  that  of 
the  apsides,  the  moon  is  sometimes  at  her 
greatest  distance  from  the  earth  at  the 
moment  when  she  passes  centrally  over  the 
sun,  and  sometimes  at  her  least  distance, 
or  she  may  be  at  any  intervening  distance. 
If  she  is  in  the  nearer  part  of  her  orbit, 
her  disk  just  covers  that  of  the  sun,  and  the 
eclipse  is  total;  if  she  is  in  the  farther  part 
(since  the  apparent  size  of  bodies  diminishes 
with  increase  of  distance),  her  disk  does  not 
entirely  cover  the  sun,  and  a  rim  of  the 
latter  is  visible  all  around  the  moon.  This 
is  called  an  annular  eclipse,  because  of  the 
ring  shape  of  the  part  of  the  sun  remaining 
visible. 

The  length  of  the  shadow  which  the  moon 
casts  toward  the  earth  during  a  solar  eclipse 
also  plays  an  important  part  in  these  phe- 
nomena. This  length  varies  with  the  distance 


168  The  Solar  System 

from  the  sun.  Since  the  moon  accompanies 
the  earth,  it  follows  that  when  the  latter 
is  in  aphelion,  or  at  its  greatest  distance  from 
the  sun,  the  moon  is  also  at  its  greatest 
mean  distance  from  the  sun,  and  the  length 
of  the  lunar  shadow  may,  in  such  circum- 
stances, be  as  much  as  236,000  miles.  When 
the  earth,  attended  by  the  moon,  is  in 
perihelion,  the  length  of  the  moon's  shadow 
may  be  only  about  228,000  miles.  The 
average  length  of  the  shadow  is  about  232,000 
miles.  This  is  nearly  7000  miles  less  than 
the  average  distance  of  the  moon  from  the 
earth,  so  it  is  evident  that  generally  the 
shadow  is  too  short  to  reach  the  earth,  and 
it  would  never  reach  it,  and  there  would 
never  be  a  total  eclipse  of  the  sun,  but  for 
the  varying  distance  of  the  moon  from  the 
earth.  When  the  moon  is  nearest  the  earth, 
or  in  perigee,  its  distance  may  be  as  small  as 
221,600  miles,  and  in  all  cases  when  near 
perigee  it  is  near  enough  for  the  shadow  to 
reach  the  earth. 

Inasmuch,  as  the  moon's  shadow,  even 
under  the  most  favourable  circumstances, 
is  diminished  almost  to  a  point  before  touch- 
ing the  earth,  it  hardly  need  be  said  that  it 
can  cover  but  a  very  small  area  on  the  earth's 


Eclipses  169 

surface.  Its  greatest  possible  diameter  can- 
not exceed  about  167  miles,  but  ordinarily  it 
is  much  smaller.  If  both  the  earth  and  the 
moon  were  motionless,  this  shadow  would  be  a 
round  or  oblong  dot  on  the  earth,  its  shape 
varying  according  as  it  fell  square  or  sloping 
to  the  surface ;  but  since  the  moon  is  continu- 
ally advancing  in  its  orbit,  and  the  earth  is 
continually  rotating  on  its  axis,  the  shadow 
moves  across  the  earth,  in  a  general  west 
to  east  direction.  But  the  precise  direction 
varies  with  circumstances,  as  does  also  the 
speed.  The  latter  can  never  be  less  than 
about  a  thousand  miles  per  hour,  and  that, 
only  in  the  neighbourhood  of  the  equator. 
The  moon  advances  eastward  about  2100 
miles  per  hour,  and  the  earth's  surface  turns 
in  the  same  direction  with  a  velocity  dimin- 
ishing from  about  a  thousand  miles  an  hour 
at  the  equator  to  o  at  the  poles.  It  is  the 
difference  between  the  velocity  of  the  earth's 
rotation  and  that  of  the  moon's  orbital 
revolution  which  determines  the  speed  of 
the  shadow.  The  greatest  time,  which  the 
shadow  can  occupy  in  passing  a  particular 
point  on  the  earth  is  only  eight  minutes,  but 
ordinarily  this  is  reduced  to  one,  two,  or 
three  minutes.  The  true  shadow  only  lasts 


170  THe  Solar  System 

during  the  time  that  the  moon  covers  the 
whole  face  of  the  sun,  but  before  and  after 
this  total  obscuration  of  the  solar  disk  the 
sun  is  seen  partially  covered  by  the  moon, 
and  these  partial  phases  of  the  eclipse  may 
be  seen  from  places  far  aside  from  the  track 
which  the  central  shadow  pursues.  It  is 
only  during  a  total  eclipse,  and  only  from 
points  situated  within  the  shadow  track,  that 
the  solar  corona  is  visible. 

In  a  lunar  eclipse  it  is  the  earth  that  is 
the  intervening  body,  and  its  shadow  falls 
upon  the  moon.  A  solar  eclipse  can  only 
occur  at  the  time  of  new  moon,  and  a  lunar 
eclipse  only  at  the  time  of  full  moon.  The 
shadow  of  the  earth  is  much  longer  and 
broader  than  that  of  the  moon,  and  it  never 
fails  to  reach  the  moon,  so  that  it  is  not 
necessary  here  to  consider  its  varying  length. 
The  width,  or  diameter,  of  the  shadow  at 
the  average  distance  of  the  moon  from  the 
earth  is  about  5700  miles.  The  moon  may 
pass  through  the  centre  of  the  shadow,  or 
to  one  side  of  the  centre,  or  merely  dip  into 
the  edge  of  it.  When  it  goes  deep  enough 
into  the  shadow  to  be  entirely  covered, 
the  eclipse  is  total;  otherwise  it  is  partial. 
Since  in  a  total  lunar  eclipse  the  entire  moon 


Eclipses  i?1 

is  covered  by  the  shadow,  it  is  evident  that 
such  an  eclipse,  unlike  a  solar  one,  may  be 
visible  simultaneously  from  all  parts  of  the 
earth  which,  at  the  time,  lie  on  the  side 
facing  the  moon.  In  other  words,  the  earth's 
shadow  does  not  make  merely  a  narrow 
track  across  the  face  of  the  moon,  but 
completely  buries  it.  When  the  moon  passes 
centrally  through  the  shadow,  she  may 
remain  totally  obscured  for  about  two  hours. 
But  the  moon  does  not  completely  dis- 
appear at  such  times,  because  the  refraction 
of  the  earth's  atmosphere  bends  a  little 
sunlight  round  its  edges  and  casts  it  into 
the  shadow.  If  the  atmosphere  round  the 
edges  of  the  earth  happens  to  be  thickly 
charged  with  clouds,  but  little  light  is  thus 
refracted  into  the  shadow,  and  the  moon 
appears  very  faint,  or  almost  entirely  dis- 
appears. But  this  is  rare,  and  ordinarily 
the  eclipsed  moon  shines  with  a  pale  cop- 
perish  light. 

The  occurrence  of  a  lunar  eclipse  is  governed 
by  similar  circumstances  to  those  affecting 
solar  eclipses.  The  lunar  ecliptic  limits, 
or  the  distance  on  each  side  of  the  node 
within  which  an  eclipse  may  occur,  vary 
from  9^°  to  12%°  in  either  direction. 


172  The  Solar  System 

Taking  all  the  various  circumstances  into 
account,  it  is  found  that  there  may  be, 
though  rarely,  seven  eclipses  in  a  year, 
two  being  of  the  moon  and  five  of  the  sun, 
and  that  the  least  possible  number  of  eclipses 
in  a  year  is  two,  in  which  case  both  will  be 
of  the  sun.  Taking  into  account  also  all 
the  various  positions  which  the  sun  and  moon 
occupy  with  regard  to  the  earth,  it  is  found 
that  there  exists  a  period  of  18  years,  n 
days,  8  hours,  at  the  return  of  which  eclipses 
of  both  kinds  begin  to  recur  again  in  the 
same  order  that  they  occur  in  the  next 
preceding  period.  This  is  called  the  saros, 
and  it  was  known  to  the  Chaldeans  2600 
years  ago. 

6.  The  Planets.  We  have  several  times 
mentioned  the  fact  that,  beside  the  earth, 
there  are  seven  other  principal  planets 
revolving  round  the  sun,  in  the  same  direction 
as  the  earth,  but  at  various  distances. 
We  shall  consider  each  of  these  in  the  order  of 
its  distance  from  the  sun. 

But  first  it  is  desirable  to  explain  briefly 
certain  so-called  "laws*'  which  govern  the 
motions  of  all  the  planets.  These  are  known 
as  Kepler's  laws  of  planetary  motion,  and 
are  three  in  number.  The  demonstration 


TKe  Planets  173 

of  their  truth  would  carry  us  beyond  the 
scope  of  this  book,  and  consequently  we 
shall  merely  state  them  as  they  are  recognised 
by  astronomers. 

ist  Law:  The  orbit  of  every  planet  is  an 
ellipse,  having  the  sun  situated  in  one  of  the 
foci. 

2d  Law:  The  radius  vector  of  a  planet 
describes  equal  areas  in  equal  times.  By 
the  radius  vector  is  meant  the  straight  line 
joining  the  planet  to  the  sun,  and  the  law 
declares  that  as  the  planet  moves  round  the 
sun,  the  area  of  space  swept  over  by  this 
line  in  any  given  time,  say  one  day,  is  equal 
to  the  area  which  it  will  sweep  over  in  any 
other  equal  length  of  time.  If  the  orbit 
were  a  circle  it  is  evident  at  a  glance  that  the 
law  must  be  true,  because  then  the  sun  would 
be  situated  in  the  centre  of  the  circle,  the 
length  of  the  radius  vector,  no  matter  where 
the  planet  might  be  in  the  orbit,  would  never 
vary,  and  the  area  swept  over  by  it  in  one  day 
would  be  equal  to  the  area  swept  over  in 
any  other  day,  because  all  these  areas  would 
be  precisely  similar  and  equal  triangles. 
But  Kepler  discovered  that  the  same  thing 
is  true  when  the  orbit  is  an  ellipse,  and  when, 
in  consequence  of  the  eccentricity  of  the 


174  THe  Solar  System 

orbit,  the  planet  is  sometimes  farther  from 
the  sun  than  at  other  times.  As  the  trian- 
gular area  swept  over  in  a  given  time  in- 
creases in  length  with  the  planet's  recession 
from  the  sun,  it  diminishes  in  breadth  just 
enough  to  make  up  the  difference  which 
would  otherwise  exist  between  the  different 
areas.  This  law  grows  out  of  the  fact  that 
the  force  of  gravitation  varies  inversely  with 
the  square  of  the  distance. 

3cUk?awT~The  squares  of  the  periods  (i.  e, 
times  of  revolution  in  their  orbits)  of  the 
different  planets  are  proportional  to  the  cubes 
of  their  mean  (average)  distances  from  the 
.sun.  The  meaning  of  this  will  be  best 
explained  by  an  example.  Suppose  one 
planet,  whose  distance  we  know,  has  a  period 
only  one-eighth  as  long  as  that  of  another 
planet,  whose  distance  we  do  not  know. 
Then  Kepler's  third  law  enables  us  to  cal- 
culate the  distance  of  the  second  planet. 
Call  the  period  of  the  first  planet  I ,  and  that 
of  the  second  8,  and  also  call  the  distance  of 
the  first  i,  since  all  we  really  need  to  know 
is  the  relative  distance  of  the  second,  from 
which  its  distance  in  miles  is  readily  deduced 
by  comparison  with  the  distance  of  the  first. 
Then,  by  the  law,  i2  :  82  :  I3  :  x3  ("x" 


The  Planets  175 

representing  the  unknown  quantity).  Now, 
this  is  simply  a  problem  in  proportion  where 
the  product  of  the  means  is  equal  to  the 
product  of  the  extremes.  But  1 2  =  i ,  and  also 
I3=i;  therefore  x3  =  82,  and  x  =  l//82  (the 
cube  root  of  the  square  of  8),  which  is  4. 
Thus  we  see  that  the  distance  of  the  second 
planet  must  be  four  times  that  of  the  first. 

This  third  law  of  Kepler  is  applied  to 
ascertain  the  distances  of  newly  discovered 
planets,  whose  periods  are  easily  ascertained 
by  simple  observation.  If  we  know  the 
distance  of  any  one  planet  by  measurement, 
we  can  calculate  the  distances  of  all  the 
others  after  observing  their  periods.  The 
law  also  works  conversely,  i.  e.  from  the 
distances  the  periods  can  be  calculated. 

We  now  take  up  the  various  planets  singly. 
The  nearest  to  the  sun,  as  far  as  known, 
is  Mercury,  its  average  distance  being  only 
36,000,000  miles.  But  its  orbit  is  so  eccen- 
tric that  the  distance  varies  from  28,500,000 
miles  at  perihelion  to  43,500,000  at  aphelion. 
In  consequence  its  speed  in  its  orbit  is  very 
variable,  and  likewise  the  amount  of  heat 
and  light  received  by  it  from  the  sun.  On 
the  average  it  gets  more  than  6^  times 


1 76  The  Solar  System 

as  much  solar  light  and  heat  as  the  earth 
gets.  But  at  perihelion  it  gets  2^/2  times 
as  much  as  at  aphelion,  and  the  time  which 
it  occupies  in  passing  from  perihelion  to 
aphelion  is  .only  six  weeks,  its  entire  year 
being  equal  to  88  of  our  days.  Being  situ- 
ated so  much  nearer  the  sun  than  the 
earth  is,  Mercury  is  never  visible  to  us 
except  in  the  morning  or  the  evening  sky, 
and  then  not  very  far  from  the  sun.  Its 
diameter  is  about  3000  miles,  but  its  mass 
is  not  certainly  known  from  lack  of  know- 
ledge of  its  mean  density.  This  lack  of 
knowledge  is  due  to  the  fact  that  Mercury 
has  no  satellite.  When  a  planet  has  a  satel- 
lite it  is  easy  to  calculate  its  density  from  its 
measured  diameter  combined  with  the  orbital 
speed  of  its  satellite.  Certain  considerations 
have  led  some  to  believe  that  the  mean  density 
of  Mercury  may  be  very  great,  perhaps  as 
great  as  that  of  lead,  or  of  the  metal  mercury 
itself.  Not  knowing  the  mass,  we  cannot 
say  exactly  what  the  weight  of  bodies  on 
Mercury  is.  We  are  also  virtually  ignorant 
of  the  condition  of  the  surface  of  this  planet, 
the  telescope  revealing  very  little  detail, 
but  it  is  generally  thought  that  it  bears  a 
considerable  resemblance  to  the  surface  of 


The  Planets  177 

the  moon.  There  is  another  way  in  which 
Mercury  is  remarkably  like  the  moon.  The 
latter,  as  we  have  seen,  always  keeps  the 
same  side  turned  toward  the  earth,  which 
is  the  same  thing  as  saying  that  it  turns  once 
on  its  axis,  while  going  once  around  the 
earth.  So  Mercury  keeps  always  the  same 
side  toward  the  sun,  making  one  rotation  on 
its  axis  in  the  course  of  one  revolution  in 
its  orbit.  Consequently,  one  side  of  Mer- 
cury is  continually  in  the  sunlight,  while  the 
opposite  side  is  continually  buried  in  night. 
There  must,  however,  be  regions  along  the 
border  between  these  two  sides,  where  the 
sun  does  rise  and  set  once  in  the  course  of  one 
of  Mercury's  years.  This  arises  from  the 
eccentricity  of  the  orbit,  and  the  consequent 
variations  in  the  orbital  velocity  of  the  planet, 
which  cause  now  a  little  of  one  edge  and  now 
a  little  of  the  other  edge  of  the  dark  hemi- 
sphere to  come  within  the  line  of  sunlight. 
(The  same  thing  occurs  with  the  moon, though 
to  a  less  degree  owing  to  the  smaller  eccen- 
tricity of  the  moon's  orbit,  which,  .however, 
is  sufficient  to  enable  us  to  see  at  one  time 
a  short  distance  round  one  side  of  the  moon 
and  at  another  time  a  short  distance  round 
the  opposite  side.)  This  phenomenon  is 

13 


178  THe  Solar  System 

known  as  libration.  Mercury  apparently 
possesses  an  atmosphere,  but  we  know 
nothing  certain  concerning  its  density. 

The  next  planet,  in  the  order  of  distance 
from  the  sun,  is  Venus,  whose  average 
distance  is  67,200,000  miles.  The  orbit 
of  Venus  is  remarkable  for  its  small  eccen- 
tricity, so  that  the  difference  between  its 
greatest  and  least  distances  from  the  sun  is 
less  than  a  million  miles.  The  period,  or 
year,  of  Venus  is  225  of  our  days.  Owing 
to  her  situation  closer  to  the  sun,  she  gets 
nearly  twice  as  much  light  and  heat  as  the 
earth  gets.  In  size  Venus  is  remarkably 
like  the  earth,  her  diameter  being  7713 
miles,  which  differs  by  only  205  miles  from 
the  mean  diameter  of  the  earth.  Her  axis 
is  nearly  perpendicular  to  the  plane  of  her 
orbit.  Her  globe  is  a  more  perfect  sphere 
than  thaj  of  the  earth,  being  very  little  flat- 
tened at  the  poles  or  swollen  at  the  equator. 
Although  Venus,  like  Mercury,  has  no 
satellite,  her  mean  density  has  been  calcu- 
lated by  other  means,  ,and  is  found  to  be 
0.89  that  of  the  earth.  From  this,  in  con- 
nection with  her  measured  diameter,  it  is 
easy  to  deduce  her  mass,  and  the  force  of 


The  Planets  179 

gravity  on  her  surface.  The  latter  comes  out 
at  about  0.85  that  of  the  earth,  i.  e.  a  body 
weighing  100  pounds  on  the  earth  would 
weigh  85  pounds  if  removed  to  Venus.  She 
possesses  an  atmosphere  denser  and  more 
extensive  than  would  theoretically  have  been 
expected — indicating,  perhaps,  a  difference 
of  constitution.  Her  atmosphere  has  been 
estimated  to  be  twice  as  dense  as  ours,  a 
great  advantage,  it  may  be  remarked,  from 
the  point  of  view  of  aeronautics.  But  this 
dense  and  abundant  atmosphere  renders 
Venus  a  very  difficult  object  for  the  telescope 
on  account  of  the  brilliance  of  its  reflection. 
In  consequence,  we  know  but  little  of  the 
surface  of  the  planet. 

One  important  result  of  this  is  that  the 
question  remains  undecided  whether  Venus 
rotates  on  her  axis  at  a  rate  closely  corre- 
sponding with  that  of  the  earth,  as  some 
observers  think,  or  whether,  as  others  think, 
she,  like  Mercury,  turns  only  once  on  her 
axis  in  going  once  round  the  sun.  The 
importance  of  the  question  in  its  bearing 
on  the  habitability  of  Venus  is  apparent, 
for  if  she  keeps  one  face  always  sunward, 
then  on  one  side  there  is  perpetual  day  and 
on  the  other  perpetual  night.  On  the 


i8o  The  Solar  System 

other  hand,  if  she  has  days  and  nights 
approximately  equal  in  length  to  those  of 
the  earth,  it  may  well  be  thought  that  she 
is  habitable  by  beings  not  altogether  unlike 
ourselves,  because  the  force  of  gravity  on  her 
surface  is  not  much  less  than  on  the  earth, 
and  her  dense  atmosphere,  filled  with  clouds, 
might  tend  to  shield  her  inhabitants  from 
the  effects  of  the  greater  amount  of  heat 
poured  upon  her  by  the  sun.  As  her  orbit 
is  inside  that  of  the  earth,  Venus,  like  Mer- 
cury, is  only  visible  either  in  the  evening 
or  in  the  morning  sky,  but  owing  to  her 
greater  actual  distance  from  the  sun,  her 
apparent  distance  from  it  in  the  sky  is 
greater  than  that  of  Mercury. 

Both  of  these  planets,  in  consequence  of 
passing  alternately  between  the  sun  and  the 
earth  and  round  the  opposite  side  of  the  sun, 
present  phases  resembling  those  of  the  moon. 
The  reader  can  explain  these  to  himself  by 
means  of  the  experiment,  before  mentioned, 
with  a  billiard  ball  and  a  lamp.  In  this 
case  let  the  observer  remain  seated  in  his 
chair  while  another  person  carries  the  ball 
round  the  lamp  in  such  a  manner  that  it 
shall  alternately  pass  between  the  lamp  and 
the  observer  and  round  the  other  side  of  the 


The  Great  Nebula  in  Orion 

Photographed  at  the  Lick  Observatory  by  J.  E.  Keeler,  with  the  Crossley 
reflector.     Exposure  one  hour. 


The  Planets  181 

lamp.  When  Venus  comes  nearly  in  line 
between  the  earth  and  the  sun,  she  becomes 
an  exceedingly  brilliant  object  in  either  the 
evening  or  the  morning  sky,  although  at 
such  times  we  see,  in  the  form  of  a  crescent, 
only  a  part  of  that  half  of  her  surface  which 
is  illuminated.  Her  increase  of  brightness 
at  such  times  is  due  to  her  greater  nearness 
to  the  earth.  When  between  the  earth  and 
the  sun  she  may  be  only  about  26,000,000 
miles  away,  while  when  she  is  on  the  other 
side  of  the  sun  she  may  be  over  160,000,000 
miles  away.  Both  Venus  and  Mercury 
when  passing  exactly  between  the  sun  and 
the  earth  are  seen,  in  the  form  of  small  black 
circles,  moving  slowly  across  the  sun's  disk. 
These  occurrences  are  called  transits,  and 
in  the  case  of  Venus  have  been  before  referred 
to.  They  are  more  frequent  with  Mercury 
than  with  Venus,  but  Mercury's  transits 
are  not  utilisable  for  parallax  observations. 
The  latest  transit  of  Venus  occurred  in 
1882,  and  there  will  not  be  another  until 
2004.  The  latest  transit  of  Mercury  occurred 
in  1907,  and  there  will  be  another  in  1914. 

The  earth  is  the  third  planet  in  order  of 
distance,    and    then    comes    Mars,    whose 


1 82  The  Solar  System 

average  distance  from  the  sun  is  141,500,000 
miles.  The  orbit  of  Mars  is  so  eccentric 
that  the  distance  varies  between  148,000,000 
and  135,000,000  miles.  Its  period  or  year 
is  about  687  of  our  days.  In  consequence 
of  its  distance,  Mars  gets,  on  the  average, 
a  little  less  than  half  as  much  light  and  heat 
as  the  earth  gets.  When  it  is  on  the  same 
side  of  the  sun  with  the  earth,  and  nearly  in 
line  with  them,  it  is  said  to  be  in  opposition. 
At  such  times  it  is  manifestly  as  near  the 
earth  as  it  can  come,  and  thus  an  opposition 
of  Mars  offers  a  good  opportunity  for  the 
telescopic  study  of  its  surface.  These  op- 
positions occur  once  in  about  780  days, 
but  they  are  not  all  of  equal  importance, 
because  the  distance  between  the  two  planets 
is  not  the  same  at  different  oppositions. 
The  cause  of  the  difference  of  distance  is 
the  eccentricity  of  the  orbit.  If  an  opposi- 
tion occurs  when  Mars  is  in  aphelion  its  dis- 
tance from  the  earth  will  be  about  61,000,000 
miles,  but  if  the  opposition  occurs  when 
Mars  is  in  perihelion  the  distance  will  be 
only  about  35,000,000  miles.  The  average 
distance  at  an  opposition  is  about  48,500,000 
miles.  The  most  favourable  oppositions 
always  occur  in  August  or  September, 


The  Planets 


183 


and   are   repeated   at   an   interval  of  from 
fifteen  to  seventeen  years.     But  at  some  of 


APHELION 


PERIHELION 


Fig.  16.     Orbits  of  Mars  and  the  Earth. 

Inspection  shows  at  once  why  the  oppositions  of  Mars 
which  occur  in  August  and  September  are  the  most 
favourable  because  Mars  being  then  near  the  perihelion 
point  of  its  elongated  orbit  is  comparatively  near  the 
earth,  while  oppositions  which  occur  in  February  and 
March  are  very  unfavourable  because  then  Mars  is  near 
the  aphelion  point  of  its  orbit,  and  its  distance  from  the 
earth  is  much  greater.  The  oppositions  occur  along  the 
more  favourable  part  of  the  orbit  about  two  years  and 
two  months  apart.  Thus  the  figure  shows  that  the 
opposition  of  September  24,  1909  was  followed  by  one 
on  November  25,  1911. 

the  intervening  oppositions  the  distance  of 
the  planet  is  not  too  great  to  afford  good 
views  of  its  surface.  The  diameter  of  Mars 


184  THe  Solar  System 

is  about  4330  miles,  with  a  similar  polar 
flattening  to  that  of  the  earth.  Its  density 
is  0.71  that  of  the  earth,  and  the  force  of 
gravity  on  its  surface  0.38.  A  body  weighing 
100  pounds  on  the  earth  would  weigh  38 
pounds  on  Mars.  The  evidence  in  regard 
to  its  atmosphere  is  conflicting,  but  the 
probability  is  that  it  has  an  atmosphere 
not  denser  than  that  existing  on  our  highest 
mountain  peaks.  Opinions  concerning  the 
existence  of  water- vapour  on  Mars  are  also 
conflicting.  One  fact  tending  to  show  that 
its  atmosphere  must  be  very  rare  and  cloud- 
less is  that  its  surface  features  are  very 
plainly  discernible  with  telescopes. 

About  each  pole,  as  it  happens  to  be  turned 
earthward,  is  to  be  seen  a  round  white  patch 
(supposed  to  be  snow),  and  this  gradually 
disappears  as  the  summer  advances  in  that 
hemisphere  of  the  planet — for  Mars  has 
seasons  very  closely  resembling  our  seasons, 
except  that  they  are  about  twice  as  long. 
The  inclination  of  the  axis  of  Mars  to  the 
plane  of  its  orbit  is  about  24°  50',  which  is 
not  very  different  from  the  inclination  of 
the  earth's  axis.  Moreover,  Mars  rotates 
in  a  period  of  24  hours,  37  min.,  22  sec., 
so  that  the  length  of  day  and  night  upon 


The  Planets  185 

its  surface  is  very  nearly  the  same  as  upon 
the  earth.  The  surface  of  the  planet  is 
marked  by  broad  irregular  areas  of  con- 
trasting colour,  or  tone,  some  of  them  being 
of  a  slightly  reddish,  or  yellowish,  hue, 
and  others  of  a  neutral  dusky  tint.  The 
general  resemblance  to  a  globe  of  the  earth, 
with  differently  shaped  seas  and  oceans, 
is  striking. 

On  account  of  the  many  likenesses  between 
Mars  and  the  earth,  some  astronomers 
are  disposed  to  think  that  Mars  may  be  a 
habitable  planet.  The  terms  ''seas"  and 
"continents"  were  formerly  applied  to  the 
contrasted  areas  just  spoken  of,  but  now  it 
is  believed  that  there  are  no  large  bodies  of 
water  on  Mars.  Crossing  the  light,  or 
reddish-coloured,  areas  there  are  sometimes 
seen  great  numbers  of  intersecting  lines, 
very  narrow  and  faint,  which  have  received 
the  name  of  "canals."  Some  speculative 
minds  find  in  these  ground  for  believing  that 
they  are  of  artificial  origin,  and  a  theory 
has  been  built  up,  according  to  which  the 
so-called  canals  are  "irrigated  bands,"  the 
result  of  the  labours  of  the  inhabitants. 
The  argument  of  the  advocates  of  this  theory 
is  put  about  as  follows:  Mars  is  evidently 


i86  THe  Solar  System 

a  nearly  dried-up  planet,  and  most  of  the 
water  left  upon  it  is  periodically  locked  up 
in  the  polar  snows.  As  these  snows  melt 
away  in  the  summer  time,  now  in  one  hemi- 
sphere and  now  in  the  other,  the  water  thus 
formed  is  conducted  off  toward  the  tropical 
and  equatorial  zones  by  innumerable  canals, 
too  small  to  be  seen  from  the  earth.  The 
lands  irrigated  by  these  canals  are  narrow 
strips,  whose  situation  is  determined  by 
local  circumstances,  and  which  cross  one 
another  in  all  directions.  Within  these 
bands,  which  enlarge  into  rounded  "oases" 
where  many  of  them  intersect,  vegetation 
pushes,  and  its  colour  causes  them  to  appear 
as  dark  lines  and  patches  on  the  surface 
of  the  planet.  The  fact  that  the  lines  make 
their  appearance  gradually,  after  the  polar 
caps  begin  to  disappear,  is  regarded  as 
strongly  corroborative  of  the  theory.  In 
answer  to  the  objection  that  works  so 
extensive  as  this  theory  of  irrigation  calls 
for  would  be  practically  impossible,  it  is 
replied  that  the  relatively  small  force  of 
gravity  on  Mars  not  only  immensely  di- 
minishes the  weight  of  all  bodies  there,  but 
also  renders  it  possible  for  animal  forms 
to  attain  a  greater  size,  with  corresponding 


The  Planets  187 

increase  of  muscular  power.  It  is  likewise 
argued  that  Mars  may  have  been  longer 
inhabited  than  the  earth,  and  that  its  in- 
habitants may  consequently  have  developed 
a  more  complete  mastery  over  -the  powers 
of  nature  than  we  as  yet  possess.  Many 
astronomers  reject  these  speculations,  and 
even  aver  that  the  lines  called  "canals" 
(and  it  must  be  admitted  that  many  powerful 
telescopes  show  few  or  none  of  them) 
have  no  real  existence,  what  is  seen,  or 
imagined  to  be  seen,  being  due  to  some 
peculiarity  of  the  soil,  rocks,  or  atmosphere. 
Mars  has  two  small  satellites,  revolving 
round  it  with  great  speed  at  close  quarters. 
The  more  distant  satellite,  Deimos,  is  14,600 
miles  from  the  centre  of  Mars  and  goes 
round  it  in  30  hours,  18  min.  The  nearer 
one,  Phobos,  is  only  5800  miles  from  the 
planet's  centre,  and  its  period  of  revolution 
is  only  7  hours,  39  min.,  so  that  it  makes 
more  than  three  circuits  while  the  planet 
is  rotating  once  on  its  axis.  Both  of  the 
satellites  are  minute  in  size,  probably  under 
ten  miles  in  diameter. 

Beyond  Mars,  at  an  average  distance  of 
about  246,000,000  miles  from  the  sun,  is  a 


i88  The  Solar  System 

system  of  little  planets  called  asteroids. 
More  than  600  are  now  known,  and  new  ones 
are  discovered  every  year,  principally  by 
means  of  photography.  Only  four  of  these 
bodies  are  of  any  considerable  size,  and  they 
were,  naturally,  the  first  to  be  discovered. 
They  are  Ceres,  diameter  477  miles;  Pallas, 
304  miles;  Vesta,  239  miles;  and  Juno,  120 
miles.  Many  of  the  others  have  a  diameter 
of  only  about  ten,  or  even,  perhaps,  as  little 
as  five,  miles.  Their  orbits  are  more  ec- 
centric than  those  of  any  of  the  large  planets, 
and  one  of  them,  Eros,  has  a  mean  distance  of 
135,000,000  miles,  and  a  least  distance  of  only 
105,000,000,  so  that  it  is  nearer  to  the  sun 
than  Mars  is.  Eros  may,  under  favourable 
circumstances,  approach  within  14,000,000 
miles  of  the  earth.  This  fact,  as  already 
mentioned,  has  been  taken  advantage  of 
for  measuring  its  distance  from  the  earth, 
from  which  the  distance  of  the  sun  may  be 
calculated  with  increased  accuracy.  Eros 
and  some  others  of  the  asteroids  seem  to  be 
of  an  irregular  or  fragmentary  form,  and 
this  has  been  used  to  support  a  theory,  which 
is  not,  however,  generally  accepted,  that  the 
asteroids  are  the  result  of  an  explosion, 
by  which  a  larger  planet  was  blown  to  pieces. 


The  Planets  189 

Sixth  in  order  of  distance  from  trie  sun 
(counting  the  asteroids  as  representing  a 
single  body)  is  the  greatest  of  all  the  planets, 
Jupiter.  His  average  distance  from  the  sun 
is  483,000,000  miles,  but  the  eccentricity 
of  his  orbit  causes  him  to  approach  within 
472,500,000  miles  at  perihelion,  and  to  recede 
to  493,500,000  miles  at  aphelion.  When 
in  opposition,  Jupiter's  mean  distance  from 
the  earth  is  390,000,000  miles.  This  gigantic 
planet  has  a  mean  diameter  of  87,380  miles, 
but  is  so  flattened  at  the  poles  and  bulged 
round  the  equator  that  the  polar  diameter 
is  only  84,570  miles,  while  the  equatorial 
diameter  is  90,190  miles,  a  difference  of 
5680  miles.  This  peculiar  form  is  doubtless 
due  to  the  planet's  swift  rotation.  The 
axis,  like  that  of  Venus,  is  nearly  perpendicu- 
lar to  the  plane  of  the  orbit.  He  makes  a 
complete  turn  on  his  axis  in  a  mean  period 
of  9  hours,  55  minutes.  The  reason  for  saying 
"a  mean  period"  will  appear  in  a  moment. 
Jupiter's  year  is  equal  to  n.86  of  our  years, 
but  it  comes  into  opposition  to  the  sun,  as 
seen  from  the  earth,  once  in  every  399  days. 

The  volume  of  Jupiter  is  about  1300  times 
that  of  the  earth,  i.e.  it  would  take  1300 
earths  rolled  into  one  to  equal  Jupiter  in 


19°  THe  Solar  System 

size.  But  its  mean  density  is  slightly  less 
than  one  quarter  of  the  earth's,  so  that  its 
mass  is  only  316  times  greater  than  the 
earth's.  The  force  of  gravity  on  its  surface 
is  2.64  times  the  earth's.  A  body  weighing 
100  pounds  on  the  earth  would  weigh  264 
pounds  on  Jupiter.  It  will  be  observed  that 
Jupiter's  mean  density  is  very  nearly  the 
same  as  that  of  the  sun,  and  we  conclude  that 
it  cannot  be  a  solid,  rigid  globe  like  the  earth. 
This  conclusion  is  made  certain  by  the  fact 
that  its  period  of  rotation  on  its  axis  is 
variable,  another  resemblance  to  the  sun. 
The  equatorial  parts  go  round  in  a  shorter 
period  than  parts  situated  some  distance 
north  or  south  of  the  equator.  It  may  be 
supposed  that  there  is  a  solid  nucleus  within, 
but  if  so,  no  direct  evidence  of  its  existence 
has  been  found. 

Nevertheless,  although  Jupiter  appears  to 
be  in  a  cloud-like  state,  it  does  not  shine 
with  light  of  its  own,  so  that  its  temperature, 
while  no  doubt  higher  than  that  of  the  earth, 
cannot  approach  anywhere  near  that  of  the 
sun.  We  do  not  know  of  what  materials 
Jupiter  is  composed,  for  spectroscopic  analy- 
sis applies  especially  to  bodies  which  shine 
with  their  own  light.  When  they  shine  only 


The  Planets  191 

by  reflected  light  received  from  the  sun, 
their  spectra  resemble  the  regular  solar 
spectrum,  except  for  the  presence  of  faint 
bands  due  to  absorption  in  the  planet's 
atmosphere.  It  may  be  that  there  are  no 
elements  of  great  atomic  density,  such  as 
iron  or  lead,  in  the  globe  of  Jupiter.  Yet 
in  the  course  of  long  ages  the  planet  may 
become  smaller  and  more  condensed,  in 
consequence  of  the  escape  of  its  internal 
heat.  In  this  way  Jupiter  may  be  regarded 
as  representing  an  intermediate  stage  of 
evolution  between  an  altogether  vaporous 
and  very  hot  body  like  the  sun,  and  a  cool 
and  solid  one  like  the  earth. 

Jupiter  presents  a  magnificent  appearance 
in  a  good  telescope.  Its  oblong  disk  is 
seen .  crossed  in  an  east  and  west  direction, 
and  parallel  to  its  equator,  by  broad,  vari- 
coloured bands,  called  belts.  These  fre- 
quently change  in  form  and,  to  some  extent, 
in  situation,  as  well  as  in  number.  But 
there  are  always  at  least  two  wide  belts, 
one  on  each  side  of  the  equator.  In  1878 
a  very  remarkable  feature  was  noticed  just 
south  of  the  principal  south  belt  of  Jupiter, 
which  has  become  celebrated  under  the  name 
of  the  Great  Red  Spot.  In  a  few  years 


192  THe  Solar  System 

after  its  discovery  its  colour  faded,  but  it 
still  remains  visible,  with  varying  degrees 
of  distinctness,  as  an  oblong  marking,  about 
30,000  miles  long  and  7000  miles  broad. 
The  outer  border  of  the  great  south  belt 
bends  away  from  the  spot,  as  if  some  force 
of  repulsion  acted  between  them,  or  as  if  the 
spot  were  an  elevation  round  which  the 
clouds  of  the  belt  flowed  like  a  river  round 
a  projecting  headland.  The  nature  of  this 
curious  spot  is  unknown.  Other  smaller 
spots,  sometimes  white,  sometimes  dusky, 
occasionally  make  their  appearance,  but 
they  do  not  exhibit  the  durability  of  the 
Great  Red  Spot. 

Jupiter  has  eight  satellites,  four  of  which, 
known  since  the  time  of  Galileo,  are  con- 
spicuous objects  in  the  smallest  telescope. 
All  but  one  of  these  four  are  larger  than  our 
moon,  while  the  other  four  are  extremely 
insignificant  in  size.  The  four  principal 
satellites  are  designated  by  Roman  numerals, 
I,  II,  III,  IV,  arranged  in  the  order  of  dis- 
tance from  the  planet.  They  also  have  names 
which  are  seldom  used.  Satellite  I  (lo) 
has  a  diameter  of  2452  miles,  and  revolves 
in  a  period  of  I  day,  18  hours,  27  min.,  35.5 
sec.,  at  a  mean  distance  of  261,000  miles; 


The  Planets  193 

II  (Europa)  is  2045  miles  in  diameter, 
and  revolves  in  3  days,  13  hours,  13  min., 
42.1  sec.,  at  a  mean  distance  of  415,000 
miles;  III  (Ganymede)  has  a  diameter  of 
3558  miles,  a  period  of  seven  days,  3  hours, 
42  min.,  33.4  sec.,  and  a  mean  distance  of 
664,000  miles;  IV  (Callisto)  is  3345  miles 
in  diameter,  has  a  period  of  16  days,  16  hours, 
32  min.,  ii.2  sec.,  and  a  mean  distance  of 
1,167,000  miles.  The  object  of  giving  the 
periods  with  extreme  accuracy  will  appear 
when  we  speak  of  the  use  made  of  obser- 
vations of  Jupiter's  satellites.  The  first  of 
the  four  small  satellites,  discovered  by 
Barnard  in  1892,  is  probably  less  than  100 
miles  in  diameter,  and  has  a  mean  distance 
of  112,500  miles,  and  a  period  of  only  II 
hours,  57  min.,  22.6  sec.  The  other  small 
satellites  are  much  more  distant  than  any 
of  the  large  ones,  the  latest  to  be  discovered, 
the  eighth,  being  situated  at  a  mean  distance 
of  about  15,000,000  miles,  but  travelling 
in  an  orbit  so  eccentric  that  the  distance 
ranges  between  10,000,000  and  20,000,000 
miles.  The  period  is  about  two  and  a  fifth 
years.  But  the  most  remarkable  fact  is 
that  this  satellite  revolves  round  Jupiter 
from  east  to  west,  a  direction  contrary  to 
13 


194  TKe  Solar  System 

that  pursued  by  all  the  others,  and  contrary 
to  the  direction  which  is  almost  universal 
among  the  rotating  and  revolving  bodies  of 
the  solar  system. 

The  large  satellites  are  very  interesting 
objects  for  the  telescope.  When  they  come 
between  the  sun  and  Jupiter  their  round  black 
shadows  can  be  plainly  seen  moving  across 
his  disk,  and  when  they  pass  round  into  his 
shadow  they  are  suddenly  eclipsed,  emerging 
after  a  time  out  of  the  other  side  of  the  shadow. 
These  phenomena  are  known  as  transits  and 
eclipses,  and  their  times  of  occurrence  are 
carefully  predicted  in  the  American  Epheme- 
ris  and  Nautical  Almanac  1  published  at 
Washington  for  the  benefit  of  astronomers 
and  navigators,  because  these  eclipses  can 
be  employed  in  comparing  local  time  with 
standard  meridian  time.  Tljey  were  form- 
erly utilised  to  determine  the  velocity  of 
light,  in  this  way : 

As  the  earth  goes  round  its  orbit  inside 
that  of  Jupiter  the  latter  is  seen  in  opposition 
to  the  sun  at  intervals  of  399  days.  When 
it  is  thus  seen  the  earth  must  be  between 
the  sun  and  Jupiter,  and  the  distance  between 
the  two  planets  is  the  least  possible.  But 
when  the  earth  has  passed  round  to  the  other 


The  Planets  195 

side  of  the  sun  from  Jupiter  this  distance 
becomes  the  greatest  possible.  The  increase 
of  distance  between  the  two  planets,  as  the 
earth  goes  from  the  nearest  to  the  farthest 
side  of  its  orbit,  is  about  186,000,000  miles. 
Now  it  was  noticed  by  the  Danish  astro- 
nomer, Roemer,  that  as  the  earth  moved 
farther  and  farther  from  Jupiter  the  times 
of  occurrence  of  the  eclipses  kept  getting 
later  and  later,  until  when  the  earth  arrived 
at  its  greatest  distance  the  eclipses  were 
about  1 6  minutes  behind  time.  He  correctly 
inferred  that  the  retardation  of  the  time  was 
due  to  the  increase  of  the  distance,  and  that 
the  1 6  minutes  by  which  the  eclipses  were 
behindhand  when  the  distance  was  greatest 
represented  the  time  taken  by  light  to  cross 
the  186,000,000  miles  of  space  by  which  the 
earth  had  increased  its  distance  from  Jupiter. 
In  other  words,  light  must  travel  186,000,000 
miles  in  about  sixteen  minutes,  from  which 
it  was  easy  to  calculate  its  speed  per  second 
—which  we  now  know  to  be  186,330  miles. 
Our  knowledge  of  the  velocity  of  light  fur- 
nishes one  of  the  means  of  calculating  the 
distance  of  the  sun. 

We   come   next   to   the   beautiful   planet 
Saturn,  whose  mean  distance  from  the  sun 


196  THe  Solar  System 

is  886,000,000  miles.  The  distance  varies 
between  911,000,000  and  861,000,000  miles. 
Saturn's  year  is  equal  to  29.46  of  our  years. 
It  comes  into  opposition  every  378  days. 
The  most  surprising  feature  of  Saturn  is 
the  system  of  immense  rings  surrounding 
it  above  the  equator.  The  globe  of  the 
planet  is  76,470  miles  in  equatorial  diameter, 
and  69,780  miles  in  polar  diameter,  a  differ- 
ence of  6690  miles,  so  that  Saturn  is  even 
more  compressed  at  the  poles  and  swollen 
at  the  equator  than  Jupiter.  The  axis  of 
rotation  is  inclined  27°  from  a  perpendicular. 
The  rings  are  three  in  number,  very  thin 
in  proportion  to  their  vast  size,  and  placed 
one  within  another  in  the  same  plane.  The 
outer  diameter  of  the  outer  ring,  called  Ring 
A,  is  about  168,000  miles.  Its  breadth  is 
about  10,000  miles.  Then  comes  a  gap, 
about  1600  miles  across,  separating  it  from 
Ring  B,  the  brightest  of  the  set.  This  is 
about  16,500  miles  broad,  and  at  its  inner 
edge  it  gradually  fades  out,  blending  with 
Ring  C,  which  is  called  the  crape,  or  gauze, 
ring,  because  it  has  a  dusky  appearance, 
and  is  so  translucent  that  the  globe  of  the 
planet  can  be  seen  through  it.  This  ring  is 
about  10,000  miles  broad,  and  its  inner  edge 


The  Planets  197 

comes  within  a  distance  of  between  9000 
and  10,000  miles  of  the  surface  of  the  planet. 
Ring  A  apparently  has  a  very  narrow  gap 
running  round  at  about  a  third  of  its  breadth 
from  the  outer  edge.  This,  known  as 
Encke's  Division,  is  not  equally  plain  at  all 
times.  Occasionally  observers  report  the 
temporary  appearance  of  other  thin  gaps. 
The  mean  density  of  Saturn  is  less  than 
that  of  any  other  planet,  being  but  0.13  that 
of  the  earth,  or  0.72  that  of  water.  It 
follows  that  this  great  planet  would  float  in 
water.  The  weight  of  bodies  at  its  surface 
would  be  a  little  less  than  three  quarters 
of  their  weight  on  the  surface  of  the  earth. 
The  globe  of  Saturn,  like  that  of  Jupiter, 
is  marked  by  belts  parallel  with  the  equator, 
but  they  are  less  definite  in  outline  and  less 
conspicuous  than  the  belts  of  Jupiter.  The 
equatorial  zone  often  shows  a  beautiful  pale 
salmon  tint,  while  the  regions  round  the 
poles  are  faintly  bluish.  Light  spots  are 
occasionally  seen  upon  the  planet,  and  it 
appears  to  rotate  more  rapidly  at  the  equator 
than  in  the  higher  latitudes.  There  seems 
to  be  every  reason  to  think  that  Saturn, 
also,  is  of  a  vaporous  constitution,  although 
it  may  have  a  relatively  condensed  nucleus. 


198  The  Solar  System 

But  while  the  globe  of  the  planet  appears 
to  be  vaporous,  the  same  is  not  true  of  the 
rings.  We  have  already  mentioned  the  fact 
that  they  are  exceedingly  thin  in  proportion 
to  their  great  size  and  width.  The  thickness 
has  not  been  determined  with  exactness, 
but  it  probably  does  not  exceed,  on  the  aver- 
age, one  hundred  miles.  There  appear  to  be 
portions  of  the  rings  which  are  thicker  than 
the  average,  as  if  the  matter  of  which  they 
are  composed  were  heaped  up  there.  This 
matter  evidently  consists  of  an  innumerable 
multitude  of  small  bodies.  In  other  words, 
the  rings  are  composed  of  swarms  of  what 
may  be  called  meteors.  That  their  com- 
position must  be  of  this  nature,  although 
the  telescope  does  not  reveal  it,  has  been 
proved  in  two  ways:  first,  by  mathematical 
calculation,  which  shows  that  if  the  rings 
were  all  of  a  piece,  whether  solid  or  liquid, 
they  would  be  destroyed  by  the  contending 
forces  of  attraction  to  which  they  are  subject ; 
and,  second,  by  spectroscopic  observation, 
which  proves,  in  a  way  that  will  be  shown 
when  we  come  to  deal  with  the  stars,  that 
the  rings  rotate  with  velocities  proportional 
to  the  distances  of  their  various  parts  from 
the  centre  of  the  planet.  Hence  it  is  inferred 


The  Planets  199 

that  they  must  consist  of  a  vast  number 
of  small  bodies  or  particles. 

Saturn  has  ten  satellites,  all  revolving 
outside  the  rings.  The  names  of  nine  of 
these  in  the  order  of  increasing  distance  are: 
Mimas,  distance  117,000  miles;  Enceladus, 
distance  157,000  miles;  Tethys,  distance 
186,000  miles;  Dione,  distance  238,000  miles; 
Rhea,  distance  332,000  miles;  Titan,  dis- 
tance 771,000  miles;  Hyperion,  distance 
934,000  miles;  Japetus,  distance  2,225,000 
miles;  and  Phcebe,  distance  8,000,000  miles. 
The  last,  like  the  eighth  satellite  of  Jupiter, 
revolves  in  a  retrograde  direction.  Only 
Titan  and  Japetus  are  conspicuous  ob- 
jects. The  period  of  Mimas  is  only  about 
22^  hours;  that  of  Titan  is  15  days,  22 
hours,  41  min.,  and  that  of  Japetus  about 
79  days,  8  hours.  Barnard's  measurements 
indicate  for  Titan  a  diameter  of  2720  miles. 
Japetus  is  probably  about  two-thirds  as 
great  in  diameter  as  Titan. 

Beyond  Saturn,  in  the  order  named,  are 
Uranus  and  Neptune.  The  mean  distance 
of  the  former  from  the  sun  is  1,782,000,000 
miles,  and  that  of  the  latter  2,791,500,000 
miles.  The  orbit  of  Uranus  is  more  eccentric 
than  that  of  Neptune.  The  diameter  of 


2OO  T"He  Solar  System 

Uranus  is  about  32,000  miles  and  that  of 
Neptune  about  35,000  miles.  The  year  of 
Uranus  is  equal  to  84  of  our  years,  and  that 
of  Neptune  to  164.78.  These  planets  are 
so  remote,  and  so  poorly  illuminated  by  the 
sun,  that  the  telescope  reveals  very  little 
detail  on  their  surfaces.  Their  density  is 
somewhat  less  than  that  of  Jupiter.  Uranus 
has  four  satellites,  Ariel,  Umbriel,  Titania,  and 
Oberon,  situated  at  the  respective  distances 
of  120,000,  167,000,  273,000,  and  365,000 
miles.  Neptune  has  one,  nameless,  satellite, 
at  a  distance  of  225,000  miles. 

The  most  remarkable  thing  about  these 
two  planets  is  that  their  axes  of  rotation, 
as  compared  with  those  of  all  the  other 
planets,  are  tipped  over  into  a  different 
plane,  so  that  they  rotate  in  a  retrograde 
or  backward  direction,  and  their  satellites, 
in  like  manner,  revolve  from  east  to  west. 
The  axis  of  Uranus  is  not  far  from  upright 
to  the  plane  of  the  ecliptic,  so  that  the  motion 
of  its  satellites  carries  them  alternately  far 
northward  and  far  southward  of  that  plane, 
but  the  axis  of  Neptune  is  tipped  so  far  over 
that  the  retrograde,  or  east  to  west,  motion 
is  very  pronounced.  Neptune  is  celebrated 
for  having  been  discovered  by  means  of 


Photographs  of  Mars 

Made  at  the  Yerkes  Observatory  by  E.  E.  Barnard,  with  the  forty-inch 
refractor,  September  28,  1909. 


Comets  201 

mathematical  calculations,  based  on  its  dis- 
turbing attraction  on  Uranus.  These  cal- 
culations showed  where  it  ought  to  be  at 
a  certain  time,  and  when  telescopes  were 
pointed  at  the  indicated  spot  the  planet  was 
found.  Similar  disturbances  of  the  motions 
of  Neptune  lead  some  astronomers  to  think 
that  there  is  another,  yet  undiscovered, 
planet  still  more  distant.  '  <&***>  *  *<***> 
7.  Comets.  Comets  are  the  most  ex- 
traordinary in  appearance  of  all  celestial 
objects  visible  to  the  naked  eye.  Great 
comets  have  been  regarded  with  terror  and 
superstitious  dread  in  all  ages  of  the  world, 
wherever  ignorance  of  their  nature  has 
prevailed.  They  have  been  taken  for  prog- 
nosticators  of  wars,  famines,  plagues,  the 
death  of  rulers,  the  outbreak  of  revolutions, 
and  the  subversion  of  empires.  One  reason 
for  this,  aside  from  their  strange  and  menac- 
ing appearance,  is,  no  doubt,  the  rarity 
of  very  great  and  conspicuous  comets.  It 
was  not  until  Newton  had  demonstrated  the 
law  of  gravitation  that  the  fact  began  to  be 
recognised  that  cc^n£ts_^e_c^rtmlled  in  their 

We  now  know  that 


they  travel  in  orbits,  frequently,  and  perhaps 
always,  elliptical,  having  the  sun  in  one  of 


2O2  Tine  Solar  System 

the  foci.  Comets  are  habitually  divided 
into  two  classes:  first,  periodical  comets, 
meaning  those  which  have  been  observed 
at  more  than  one  return  to  perihelion;  and, 
second,  non-periodical  comets,  meaning  those 
which  have  been  seen  but  once,  but  which, 
nevertheless,  may  return  to  perihelion  in  a 
period  so  long  that  a  second  return  has  not 
been  observed.  A  better  division  is  into 
comets  of  short  period,  and  comets  of  long, 
or  unknown,  periods. 

Still,  many  astronomers  are  disposed  to 
think  that  the  majority  of  comets  do  not 
travel  in  elliptical  but  in  parabolic,  and  a 
few  in  hyperbolic,  orbits.  This  calls  for  a  few 
words  of  explanation.  Ellipses,  parabolas, 
and  hyperbolas  are  all  conic-section  curves, 
but  the  ellipse  alone  returns  into  itself,  or 
forms  a  closed  circuit.  In  each  case  the  sun 
is  situated  at  the  focus  where  the  perihelion, 
or  nearest  approach,  of  the  comet  occurs, 
but  only  comets  travelling  in  elliptical 
orbits  return  again  after  having  once  been 
seen.  A  comet  moving  in  a  parabola  would 
go  back  into  the  depths  of  space  nearly  in 
the  direction  from  which  it  had  come,  and 
would  never  be  seen  again ;  and  if  it  moved  in 
a  hyperbola  it  would  go  off  toward  another 


Comets 


203 


quarter  of  the  celestial  sphere,  and  likewise 
would  never   return.     Now  it   is   true  that 


Fig.  17.     Ellipse,  Parabola,  and  Hyperbola. 

The  figure  shows  graphically  why  it  is  so  difficult  to  tell 
exactly  the  form  of  a  comet's  orbit.  The  three  kinds  of 
curves  are  nearly  of  the  same  form  near  the  focus  (the 
Sun),  and  it  is  only  in  that  part  of  its  orbit  that  the 
comet  can  be  seen.  Moreover  a  comet  is.^at  best,  a 
misty  and  indefinite  object,  which  renders  it  so  much 
the  more  difficult  to  obtain  good  observations  of  its 
precise  position  and  movement. 

the  forms  calculated  for  the  orbits  of  the 
majority  of  comets  that  have  been  observed 
appear  to  be  parabolic  (a  very  few  seem 
to  be  hyperbolic),  and  if  this  is  the  fact  such 


2O4  THe  Solar  System 

comets  cannot  be  permanent  members  of 
the  solar  system,  but  must  enter  it  from  far- 
off  regions  of  space,  and  having  visited  the 
sun  must  return  to  such  regions  without  any 
tendency  to  come  back  again.  In  that  case 
they  may  pay  similar  visits  to  other  suns. 
But  it  is  quite  possible  that  what  appear 
to  be  parabolic  orbits  may,  in  reality,  be 
ellipses  of  very  great  eccentricity.  The 
difficulty  in  determining  the  precise  shape  of 
a  comet's  orbit  arises  from  the  fact  that  all 
three  of  the  curves  just  mentioned  closely 
approximate  to  one  another  in  the  neigh- 
bourhood of  their  common  focus,  the  sun, 
and  it  is  only  in  that  part  of  their  orbits 
that  comets  are  visible.  The  whole  question 
is  yet  in  abeyance,  but,  as  we  have  said, 
it  seems  likely  that  all  comets  really  move 
in  elliptical  orbits,  and  consequently  never 
get  entirely  beyond  the  control  of  the  sun's 
attraction.  But  in  all  cases  the  orbits  of 
comets  are  much  more  eccentric  than  those 
of  the  large  planets.  The  famous  comet  of 
Halley,  for  instance,  which  has  the  longest 
period  of  any  of  the  known  periodical  class, 
about  seventy-five  years,  is  3,293,000,000 
miles  from  the  sun  when  in  aphelion,  and 
only  54,770,000  miles  when  in  perihelion. 


Comets  205 

Comets,  when  near  the  sun,  are  greatly 
affected  by  the  disturbing  attraction  of 
large  planets,  and  especially  of  the  most 
massive  of  them  all,  Jupiter.  The  effect  of 
this  disturbance  is  to  change  the  form  of 
their  orbits,  with  the  not  infrequent  result 
that  the  latter  are  altered  from  apparent 
parabolas  into  unquestionable  ellipses,  and 
thus  the  comets  concerned  are  said  to  be 
"captured,"  or  made  prisoners  to  the  sun, 
by  the  influence  of  the  disturbing  planet. 
About  twenty  small  comets  are  known  as 
"Jupiter's  Comet  Family,"  because  they 
appear  to  have  been  "captured"  in  this  way 
by  him.  A  few  others  are  believed  to  have 
been  similarly  captured  by  Saturn,  Uranus, 
and  Neptune. 

The  orbits  of  comets  differ  from  those  of 
the  planets  in  other  ways  beside  their  greater 
eccentricity.  The  planets  all  move  round  the 
sun  from  west  to  east,  but  comets  move  in 
both  directions.  The  orbits  of  the  planets, 
with  the  exception  of  some  of  the  asteroids, 
all  lie  near  one  common  plane,  but  those  of 
comets  are  inclined  at  all  angles  to  this  plane, 
some  of  them  coming  down  from  the  north 
side  of  the  ecliptic  and  others  up  from  the 
south  side. 


206  THe  Solar  System 

A  comet  consists  of  two  distinct  portions : 
first,  the  head,  or  nucleus;  and,  second,  the 
tail.  The  latter  only  makes  its  appearance 
when  the  comet  is  drawing  near  the  sun,  and, 
as  a  whole,  it  is  always  directed  away  from 
the  sun,  but  usually  more  or  less  curved 
backward  along  the  comet's  course,  as  if 
the  head  tended  to  run  away  from  it.  The 
appearance  of  a  comet's  tail  at  once  suggests 
that  it  is  produced  by  some  repulsive  force 
emanating  from  the  sun.  Recently  there 
has  been  a  tendency  to  explain  this  on  the 
principle  of  what  is  known  as  the  pressure 
of  light.  This  demands  a  brief  explanation. 
Light  is  believed  to  be  a  disturbance  of  the 
universal  ether  in  the  form  of  waves  which 
proceed  from  the  luminous  body.  These 
waves  possess  a  certain  mechanical  energy 
tending  to  drive  away  bodies  upon  which 
they  impinge.  The  energy  is  relatively 
slight,  and  in  ordinary  circumstances  pro- 
duces no  perceptible  effect,  but  when  the 
body  acted  upon  by  the  light  is  extremely 
small  the  pressure  may  become  so  great 
relatively  to  gravitation  as  to  prevail  over 
the  latter.  To  illustrate  this,  let  us  recall 
two  facts — first,  that  gravitation  acts  upon 
the  mass,  i.e.  all  the  particles  of  a  body 


Comets  207 

throughout  its  entire  volume;  and,  second, 
that  pressure  acts  only  upon  the  exterior 
surface.  Consequently  gravitation  is  pro- 
portional to  the  volume,  while  the  pressure 
of  light  is  proportional  to  the  surface  of 
the  body  acted  upon.  Now  the  mass,  or 
volume,  of  any  body  varies  as  the  cube  of 
its  diameter,  and  the  surface  only  as  the 
square.  If,  then,  we  have  two  bodies,  one 
of  which  has  twice  the  diameter  of  the 
other,  the  mass  of  the  second  wjll  be  eight 
times  less  than  that  of  the  first,  but  the 
surface  will  be  only  four  times  less.  If  the 
second  has  only  one-third  the  diameter  of 
the  first,  then  its  mass  will  be  twenty-seven 
times  less,  but  its  surface  only  nine  times 
less.  Thus  we  see  that  as  we  diminish  the 
size  of  the  body,  the  mass  falls  off  more 
rapidly  than  the  area  of  the  surface,  and 
consequently  the  pressure  gains  relatively 
to  the  gravitation.  Experiment  has  cor- 
roborated the  conclusions  of  mathematics 
on  this  subject,  and  has  shown  that  when  a 
particle  of  matter  is  only  about  one  one- 
hundred-thousandth  of  an  inch  in  diameter 
the  pressure  of  light  upon  it  becomes  greater 
than  the  force  of  gravitation,  and  such  a 
particle,  situated  in  open  space,  would  be 


208  The  Solar  System 

driven  away  from  the  sun  by  the  light 
waves.  This  critical  size  would  vary  with 
the  density  of  the  matter  composing  the 
particle,  but  what  we  have  said  will  serve  to 
convey  an  idea  of  its  minuteness. 

Now  in  applying  this  to  comets'  tails 
it  is  only  necessary  to  remark  that  they  are 
composed  of  either  gaseous  or  dusty  particles, 
or  both,  rising  from  the  nucleus,  probably 
under  the  influence  of  the  heat  or  the  electrical 
action  of  the  sun,  and  these  particles,  being 
below  the  critical  size,  are  driven  away  from 
the  sun,  and  appear  in  the  form  of  a  tail 
following  the  comet.  It  may  be  added  that 
the  same  principle  has  been  evoked  to  ex- 
plain the  corona  of  the  sun,  which  may  be 
composed  of  clouds  of  gas  or  dust  kept  in 
suspension  by  the  pressure  of  light. 

The  nuclei  of  comets  contain  nearly 
their  whole  mass.  The  actual  mass  of  no 
comet  is  known,  but  it  can  in  no  case  be  very 
great.  Moreover,-  it  is  probable  that  the 
nucleus  of  a  comet  does  not  consist  of  a 
single  body,  either  solid  or  liquid,  but  is 
composed  of  a  large  number  of  separate 
small  bodies,  like  a  flock  of  meteors,  crowded 
together  and  constantly  impinging  upon  one 
another.  As  the  comet  approaches  the  sun 


Cornets  209 

the  nucleus  becomes  violently  agitated,  and 
then  the  tail  begins  to  make  its  appearance. 
The  possibility  exists  of  an  encounter 
between  the  earth  and  the  head  of  a  comet, 
but  no  such  occurrence  is  known.  Two  or 
three  times,  however,  the  earth  is  believed 
to  have  gone  through  the  tail  of  a  comet, 
the  last  time  in  1910,  when  Halley's  comet 
passed  between  the  earth  and  the  sun, 
but  no  certain  effects  have  been  observed 
from  such  encounters.  The  spectroscope 
shows  that  comets  contain  various  hydro- 
carbons, sodium,  nitrogen,  magnesium,  and 
possibly  iron,  but  we  know,  as  yet,  very 
little  about  their  composition.  The  presence 
of  cyanogen  gas  was  reported  in  Halley's 
comet  at  its  last  appearance.  We  are  still 
more  ignorant  of  the  origin  of  comets.  We 
do  know,  however,  that  they  tend  to  go  to 
pieces,  especially  those  which  approach  very 
close  to  the  sun.  The  great  comet  of  1882, 
which  almost  grazed  the  sun,  was  afterward 
seen  retreating  into  space  scattered  into 
several  parts,  each  provided  with  a  tail. 
In  at  least  one  case,  several  comets  have 
been  found  travelling  in  the  same  track, 
an  indication  that  one  large  original  comet 
has  been  separated  into  three  or  four  smaller 
14 


210  THe  Solar  System 

ones.  This  appears  to  be  true  of  the  comets 
of  1843,  1880,  and  1882, — and  perhaps  the 
comet  of  1576  should  be  added.  But  the 
most  remarkable  case  of  disruption  is  that  of 
Biela's  Comet,  which  first  divided  into  two 
parts  in  1846  and  then  apparently  became 
scattered  into  a  swarm  of  meteors  which  was 
encountered  by  the  earth  in  1872,  when  it 
passed  near  the  old  track  of  the  comet. 
This  leads  us  to  our  next  subject. 

8.  Meteors.  Everybody  must,  at  some 
time,  have  beheld  the  phenomenon  known  as 
a  falling,  or  shooting,  star.  A  few  of  these 
objects  can  be  'seen  darting  across  the  sky 
on  almost  any  clear  night  in  the  course  of 
an  hour  or  two  of  watching.  Sometimes 
they  appear  more  numerously,  and  at  in- 
tervals they  are  seen  in  "showers."  They 
are  called  meteors,  and  it  is  believed  that 
they  are  minute  solid  bodies,  perhaps  aver- 
aging but  a  small  fraction  of  an  ounce  in 
weight,  which  plunge  into  the  atmosphere 
with  velocities  varying  from  twenty  to 
thirty  or  more  miles  per  second,  and  are 
set  afire  and  consumed  by  the  heat  of  friction 
developed  by  their  rush  through  the  air. 
Anybody  who  has  seen  a  bullet  melted  by 
the  heat  suddenly  developed  when  it  strikes 


Meteors  211 

a  steel  target  has  had  a  graphic  illustration 
of  the  transformation  of  motion  into  heat. 
But  if  we  could  make  the  bullet  move  fast 
enough  it  would  melt  in  the  air,  the  heat 
being  developed  by  ^Jj^r£^gjant  friction. 
The  connection  of ^§?ereo«3  with  comets 
is  very  interesting.  In  the  year  1833,  a 
magnificent  and  imposing  display  of  meteors, 
which,  for  hours,  on  the  night  between  the 
1 3th  and  I4th  of  November,  filled  the  sky 
with  fire-balls  and  flaming  streaks,  astonished 
all  beholders  and  filled  mariv  with  terror. 
It  was  found  that  these  i5Mi:ee£§  travelled 
in  an  orbit  intersecting  that  of  the  earth  at 
the  point  where  the  latter  arrived  in  the 
middle  of  November,  and  also  that  they  had 
a  period  of  revolution  about  the  sun  of  33^ 
years,  and  were  so  far  scattered  along  their 
orbit  that  they  required  nearly  three  years 
to  pass  the  point  of  intersection  with  the 
orbit  of  the  earth.  Thus  it  was  concluded 
that  for  three  years  in  succession,  in  mid- 
November,  there  should  be  a  display  of  the 
meteors  plunging  into  the  earth'.s  atmos- 
phere. But  only  in  the  year  when  the 
thickest  part  of  the  swarm  was  encountered 
by  the  earth  would  the  display  be  very 
imposing.  Upon  this  it  was  predicted  that 


212  THe  Solar  System 

there  would  be  a  recurrence  of  the  phenomenon 
of  1833  in  the  year  1866.  It  happened  as 
predicted,  except  that  the  number  of  meteors 
was  not  quite  so  great  as  before.  In  the 
meantime,  it  had  been  discovered  that  these 
meteors  followed  in  the  track  of  a  comet 
known  as  Temple's  Comet,  and  also  that 
certain  other  meteors,  which  appear  every 
year  in  considerable  numbers  about  the  loth 
of  August,  followed  the  track  of  another 
comet  called  Tut  tie's  Comet.  Then  in 
1872  came  the  display,  mentioned  in  the 
last  section,  of  meteors  which  were  evidently 
the  debris  of  the  vanished  comet  of  Biela. 
The  inference  from  so  many,  similar  cases 
was  irresistible  that  the /?Sffi^Cniust  be 
/fragments  of  destroyed  or  partially  destroyed 
comets.  Several  other  cases  of  identity  of 
orbits  between  meteors  and  comets  have 
been  discovered. 

It  has  been  said  that  the  August  meteors 
appear  every  year.  The  explanation  of  this 
is  that  they  have,  in  the  course  of  many 
ages,  been  scattered  around  the  whole  circuit 
of  their  orbit,  so  that  each  year,  about 
the  loth  of  August,  when  the  earth  crosses 
their  track,  some  of  the  meteors  are  en- 
countered. They  are  like  an  endless  rail- 


Meteors  213 

road  train  travelling  upon  a  circular  track. 
The  November  meteors  also  appear,  in  small 
numbers,  every  year,  a  fact  indicating  that 
some  of  them,  too,  have  been  scattered  all 
around  their  orbit,  although  the  great  mass 
of  them  is  still  concentrated  in  an  elongated 
swarm,  and  a  notable  display  can  only  occur 
when  this  swarm  is  at  the  crossing  simul- 
taneously with  the  earth.  These  meteors 
were  eagerly  awaited  in  1899,  when  it  was 
hoped  that  the  splendid  displays  of  1833  and 
1866  might  be  repeated,  but,  unfortunately, 
in  the  meantime  the  planets  Jupiter  and 
Saturn,  by  their  disturbing  attractions,  had 
so  altered  the  position  of  the  path  of  the 
meteors  in  space  that  the  principal  swarm 
missed  the  connection.  There  are  many 
other  periodical  meteor  showers,  generally 
less  brilliant  than  those  already  mentioned, 
and  some  astronomers  think  that  all  of  them 
had  their  origin  from  comets. 

It  is  not  known  that  any  meteor  from  any 
of  these  swarms  has  ever  reached  the  surface 
of  the  earth.  The  meteors  appear  to  be  so 
small  that  they  are  entirely  burnt  up  before 
they  can  get  through  the  atmosphere,  which 
thus  acts  as  a  shield  against  these  little 
missiles  from  outer  space.  But  there  is 


214  THe  Solar  System 

another  class  of  meteoric  bodies,  variously 
known  as  meteorites,  aerolites,  uranoliths,  or 
bolides,  which  consists  of  larger  masses, 
and  these  sometimes  fall  upon  the  earth, 
after  a  fiery  passage  through  the  air. 
Specimens  of  them  may  be  seen  in  many 
museums.  They  are  divided  into  two 
principal  classes,  according  to  their  compo- 
sition: first,  stony  meteorites,  which  are  by 
far  the  most  numerous;  and,  second,  iron 
meteorites,  which  consist  of  almost  pure  iron, 
generally  alloyed  with  a  little  nickel.  The 
stony  meteorites,  which  usually  contain 
some  compound  of  iron,  consist  of  a  great 
variety  of  substances,  including  between 
twenty  and  thirty  different  chemical  elements. 
Although  they  resemble  in  many  ways 
minerals  of  volcanic  origin  on  the  earth, 
they  also  possess  certain  characteristics  by 
which  they  can  be  recognised  even  when 
they  have  not  been  seen  to  fall. 

When  a  meteorite  passes  through  the  air 
it  makes  a  brilliant  display  of  light,  and 
frequently  bursts  asunder,  with  a  tremendous 
noise,  scattering  its  fragments  about.  The 
largest  fragment  of  a  meteorite  actually 
seen  to  fall,  weighs  about  a  quarter  of  a  ton. 
Upon  striking  the  ground  the  meteorite 


Meteors  215 

sometimes  penetrates  to  a  depth  of  several 
feet,  and  some  have  been  picked  up  which 
were  yet  hot  on  the  surface,  although  very 
cold  within.  It  is  not  known  that  meteorites 
have  any  connection  with  comets,  and  their 
origin  can  only  be  conjectured.  Among 
the  various  suggestions  that  have  been  made 
the  following  may  be  mentioned:  (i)  that 
they  have  been  shot  out  of  the  sun — par- 
ticularly the  iron  meteorites;  (2)  that  they 
were  cast  into  space  by  lunar  volcanoes 
when  the  moon  was  still  subject  to  volcanic 
action;  (3)  that  they  are  the  products 
of  explosion  in  the  stars.  But  some  astro- 
nomers are  disposed  to  think  that  they 
originated  in  a  similar  manner  to  other 
members  of  the  solar  system,  although  it  is 
difficult,  on  this  hypothesis,  to  account  for 
their  great  density.  The  opinion  that  the 
iron  meteorites  have  come  from  the  sun, 
or  some  other  star,  is  enforced  by  the  fact 
that  they  contain  hydrogen,  carbon,  and 
helium,  in  forms  suggesting  that  these  gases 
were  absorbed  while  the  bodies  were  immersed 
in  a  hot,  dense  atmosphere. 


PART  IV. 

THE  FIXED  STARS. 


217 


PART  IV. 

THE  FIXED  STARS. 

i.  The  Stars.  The  stars  are  distant 
suns,  varying  greatly  in  remoteness,  in 
magnitude,  and  in  condition.  Many  of 
them  are  much  smaller  than  our  sun,  and 
many  others  are  as  much  larger.  They  vary, 
likewise  in  age,  or  state  of  development. 
Some  are  relatively  young,  others  in  a 
middle  stage,  and  still  others  in  a  condition 
that  may  be  called  solar  decrepitude. 
These  proofs  of  evolution  among  the  stars, 
the  knowledge  of  which  we  owe  mainly 
to  spectroscopic  analysis,  serve  to  establish 
more  firmly  the  conclusion,  to  which  the 
simple  aspect  of  the  heavens  first  leads  us, 
that  the  universe  is  a  connected  system, 
governed  everywhere  by  similar  laws  and 
consisting  of  like  materials. 

The  number  of  stars  visible  to  the  naked 

eye  is  about  six  thousand,  but  telescopes  show 

tens  of  millions.    It  is  customary  to  divide  the 

stars  into  classes,  called  magnitudes,  accord- 

219 


220  The  Fixed  Stars 

ing  to  their  apparent  brightness.  By  a 
system  of  photometry,  or  light-measurement, 
they  are  grouped  into  stars  of  the  first, 
second,  third,  etc.,  magnitude.  With  the 
naked  eye  no  stars  fainter  than  the  sixth 
magnitude  are  visible,  but  very  powerful 
telescopes  may  show  them  down  to  the 
eighteenth  magnitude.  Each  magnitude  is 
about  two  and  a  half  times  brighter  than  the 
next  magnitude  below  in  the  scale.  A  first- 
magnitude  star  is  about  one  hundred  times 
brighter  than  one  of  the  sixth  magnitude. 
But,  in  reality,  the  variation  of  brightness 
is  gradual,  and  for  very  accurate  estimates 
fractions  of  a  magnitude  have  to  be  employed. 
There  are  about  twenty  first-magnitude  stars, 
but  they  are  not  all  of  equal  brightness. 
A  more  accurate  photometry  assumes  a 
zero  magnitude,  very  nearly,  represented 
by  the  star  Arcturus,  and  makes  the  ratio 
2.512.  Thus  a  star,  nearly  represented  by 
Aldebaran  or  Altair,  which  is  2.512  times 
fainter  than  the  zero  magnitude,  is  of  the 
first  magnitude,  and  a  star,  nearly  repre- 
sented by  the  North  Star,  which  is  2.512 
times  fainter  than  the  first  magnitude,  is  of 
the  second  magnitude.  Counting  in  the 
other  direction,  a  star,  like  Sirius,  which  is 


The  Stars  221 

brighter  than  the  zero  magnitude,  is  said  to 
be  of  a  negative  magnitude.  The  magnitude 
of  Sirius  is  --  1.6.  There  is  only  one  other 
star  of  negative  magnitude,  Canopus,  whose 
magnitude  is  —  0.9.  But  for  ordinary  pur- 
poses one  need  not  trouble  himself  with  these 
refinements. 

The  stars  are  divided  into  five  principal 
types,  according  to  their  spectra.  These 
are: 

I.  White  stars,  having  a  bluish  tinge,  in 
which  the  spectrum  is  characterised  by  broad 
dark  bands,  due  apparently  to  an  extensive 
atmosphere   of   hydrogen,    while    there   are 
but    few    lines    indicating    the    presence    of 
metallic    vapours.     About    half    the    stars 
whose  spectra  have  been  studied  belong  to 
Type  I. 

II.  Yellowish- white  stars,  resembling  the 
sun  in  having  their  spectra  crossed  with  a 
great  number  of  lines  produced  by  metallic 
vapours,  while  the  hydrogen  lines  are  less 
conspicuous.     These  are  often  called  solar 
stars,  and  they,  too,  are  very  numerous. 

III.  Orange   and   slightly  reddish   stars, 
whose  spectra  contain  mostly  broad  bands 
instead    of   narrow   lines,    the   bands   being 
situated  toward  the  blue  end  of  the  spectrum, 


222  TKe  Tixed  Stars 

whence  the  prevailing  colour,  since  the  blue 
light  is  thus  cut  off.  Only  a  few  hundred 
of  these  stars  are  known,  but  they  include 
most  of  the  well-known  variable  stars. 

IV.  Small    deep-red    stars    having    dark 
bands  absorbing  the  light  of  the  red  end  of 
the    spectrum.     Less    than    a    hundred    of 
these  stars  are  known. 

V.  Stars  whose  spectra  are  characterised 
by  bright  instead  of  dark  lines,   although 
they    also    show    dark   bands.     The   bright 
lines  indicate  that  the  atmospheric  vapours 
producing  them  are  at  a  higher  temperature 
than  the  body  of  the  star.     Stars  of  this 
type  are  sometimes  called  Wolf-Rayet  stars 
and  they  are  few  in  number. 

Various  modifications  of  these  main  types 
exist,  but  we  cannot  here  enter  into  an 
account  of  them.  In  a  general  way,  al- 
though there  are  exceptions  depending  upon 
the  precise  nature  of  each  spectrum,  the 
white  stars  are  thought  to  be  younger  than 
the  yellowish  ones,  and  the  red  stars  older. 

In  speaking  of  the  "size"  of  the  stars  we 
really  mean  their  luminosity,  or  the  amount 
of  light  radiated  from  them.  When  a  star 
is  said  to  be  a  thousand  times  greater  than 
the  sun,  the  meaning  is  that  the  amount  of 


The  Stars  223 

light  that  it  gives  would,  if  both  were  viewed 
from  the  same  distance,  be  equal  to  a 
thousand  times  the  amount  given  by  the 
sun.  We  have  no  direct  knowledge  of  the 
actual  size  of  the  stars  as  globes,  because 
the  most  powerful  telescope  is  unable  to 
reveal  the  real  disk  of  a  star.  In  comparing 
the  luminosity  of  a  star  with  that  of  the  sun 
its  distance  must  be  taken  into  account. 
Most  of  the  stars  are  so  far  away  that  we 
really  know  nothing  of  their  distances,  but 
there  are  fifty  or  more  which  lie  within  a 
distance  not  too  great  to  enable  us  to  obtain 
an  approximate  idea  of  what  it  is.  The 
nearest  star  in  the  northern  sky  is  so  far 
from  being  the  brightest  that  it  can  barely 
be  seen  with  the  naked  eye.  It  must  be 
very  much  less  luminous  than  the  sun.  On 
the  other  hand,  some  very  bright  stars  lie 
at  a  distance  so  immense  that  it  can  hardly 
be  estimated,  and  they  must  exceed  the  sun 
in  luminosity  hundreds  and  even  thousands 
of  times. 

The  question  of  the  distance  of  the  stars 
has  already  been  treated  in  the  section  on 
Parallax.  In  employing  our  knowledge  of 
star  distances  for  the  purpose  of  comparing 
their  luminosity  with  that  of  the  sun,  we 


224  THe  Fixed  Stars 

must  first  ascertain,  as  accurately  as  possible, 
the  actual  amount  of  light  that  the  star 
sends  to  the  earth  as  compared  with  the 
actual  amount  of  light  that  the  sun  sends. 
The  star  Arcturus  gives  to  our  eyes  about 
one  forty-billionth  as  much  light  as  the  sun 
does.  Knowing  this,  we  must  remember 
that  the  intensity  of  light  varies,  like  gravi- 
tation, inversely  as  the  square  of  the  distance. 
Thus,  if  the  sun  were  twice  as  far  away  as  it 
is,  the  amount  of  its  light  received  on  the 
earth  would  be  reduced  to  one  fourth,  and 
if  its  distance  were  increased  three  times,  the 
amount  would  be  reduced  to  one  ninth. 
If  the  sun  were  200,000  times  as  far  away, 
its  light  would  be  reduced  to  one  forty-bil- 
lionth, or  the  same  as  that  of  Arcturus. 
At  this  point  the  actual  distance  of  Arcturus 
enters  into  the  calculation.  If  that  distance 
were  200,000  times  the  sun's  distance,  we 
should  have  to  conclude  that  Arcturus 
was  exactly  equal  to  the  sun  in  luminosity, 
since  the  sun,  if  removed  to  the  same  distance, 
would  give  us  the  same  amount  of  light. 
But,  in  fact,  we  find  that  the  distance  of 
Arcturus,  instead  of  being  200,000  times  that 
of  the  sun,  is  about  10,000,000  times.  In 
other  words,  it  is  fifty  times  as  far  away  as 


The  Stars  225 

the  sun  would  have  to  be  in  order  that  it 
should  appear  to  our  eyes  no  brighter  than 
Arcturus.  From  this  it  follows  that  the 
real  luminosity  of  Arcturus  must  be  the 
square  of  50,  or  2500,  times  that  of  the  sun. 
In  the  same  manner  we  find  that  Sirius, 
which  to  the  eye  appears  to  be  the  brightest 
star  in  the  sky  (much  brighter  than  Arcturus 
because  much  nearer),  is  about  thirty  times 
as  luminous  as  the  sun. 

Many  of  the  stars  are  changeable  in 
brightness,  and  those  in  which  the  changes 
occur  to  a  notable  extent,  and  periodically, 
are  known  as  variable  stars.  It  is  probable 
that  all  the  stars,  including  the  sun,  are 
variable  to  a  slight  degree.  Among  the 
most  remarkable  variables  are  Mira,  or 
Omicron  Ceti,  in  the  constellation  Cetus, 
which  in  the  course  of  about  331  days  rises 
from  the  ninth  to  the  third  magnitude  and 
then  falls  back  again  (the  maxima  of  bright- 
ness are  irregular) ;  and  Algol,  or  Beta  Persei, 
in  the  constellation  Perseus,  which,  in  a 
period  of  2  days,  20  hours,  49  minutes, 
changes  from  the  third  to  the  second  magni- 
tude and  back  again.  In  the  case  of  Mir  a 
the  cause  of  the  changes  is  believed  to  lie 
in  the  star  itself,  and  they  may  be  connected 
is 


226  THe  Fixed  Stars 

with  its  gradual  extinction.  The  majority 
of  the  variable  stars  belong  to  this  class. 
As  to  Algol,  the  variability  is  apparently 
due  to  a  huge  dark  body  circling  close  around 
the  star  with  great  speed,  and  periodically 
producing  partial  eclipses  of  its  light.  There 
are  a  few  other  stars  with  short  periods  of 
variability  which  belong  to  the  class  of  Algol. 
When  examined  with  telescopes  many  of 
the  stars  are  found  to  be  double,  triple,  or 
multiple.  Often  this  arises  simply  from  the 
fact  that  two  or  more  happen  to  lie  in  nearly 
the  same  line  of  sight  from  the  earth,  but 
in  many  other  cases  it  is  found  that  there 
is  a  real  connection,  and  that  the  stars  con- 
cerned revolve,  under  the  influence  of  their 
mutual  gravitation,  round  a  common  centre 
of  force.  When  two  stars  are  thus  connected 
they  are  called  a  binary.  The  periods  of 
revolution  range  from  fifty  to  several 
hundred  years.  Among  the  most  celebrated 
binary  stars  are  Alpha  Centauri,  in  the 
southern  hemisphere,  the  nearest  known 
star  to  the  solar  system,  whose  components 
revolve  in  a  period  of  about  eighty  years; 
Gamma  Virginis,  in  the  constellation  Virgo, 
period  about  one  hundred  and  seventy  years ; 
and  Sirius,  period  about  fifty-three  years. 


TKe  Stars  227 

In  the  case  of  Serins,  one  of  the  components, 
although  perhaps  half  as  massive  as  its  com- 
panion, is  ten  thousand  times  less  bright. 

There  is  another  class  of  binary  stars, 
in  which  one  of  the  companions  is  invisible, 
its  presence  being  indicated  by  the  effects 
of  its  gravitational  pull  upon  the  other. 
Algol  may  be  regarded  as  an  example  of 
this  kind  of  stellar  association.  But  there 
are  stars  of  this  class,  where  the  companion 
causes  no  eclipses,  either  because  it  is  not 
dark,  or  because  it  never  passes  over  the 
other,  as  seen  from  the  earth,  but  where  its 
existence  is  proved,  in  a  very  interesting  way, 
by  the  spectroscope.  In  these  stars,  called 
spectroscopic  binaries,  two  bright  components 
are  so  close  together  that  no  telescope  is 
able  to  make  them  separately  visible,  but 
when  their  plane  of  revolution  lies  nearly 
in  our  line  of  sight  the  lines  in  their  combined 
spectrum  are  seen  periodically  split  asunder. 
To  understand  this,  we  must  recall  the 
principles  underlying  spectroscopic  analysis 
and  add  something  to  what  was  said  before 
on  that  subject.  * 

Light  consists  of  waves  in  the  ether  of 
different  lengths  and  making  upon  the  eye 
different  impressions  of  colour  according 


228  The  Fixed  Stars 

to  the  length  of  the  waves.  The  longest 
waves  are  at  the  red  end  of  the  spectrum 
and  the  shortest  at  the  blue,  or  violet,  end. 
But  since  they  all  move  onward  with  the 
same  speed,  it  is  clear  that  the  short  blue 
waves  must  fall  in  quicker  succession  on 
the  retina  of  the  eye  than  the  long  red  waves. 
Now  suppose  that  the  source  of  light  from 
which  the  waves  come  is  approaching  very 
swiftly;  it  is  easy  to  see  that  all  the  waves 
will  strike  the  eye  with  greater  rapidity, 
and  that  the  whole  spectrum  will  be  shifted 
toward  the  blue,  or  short-wave,  end.  The 
Fraunhofer  lines  will  share  in  this  shifting 
of  position.  Next  suppose  that  the  source 
of  the  light  is  retreating  from  the  eye.  The 
same  effect  will  occur  in  a  reversed  sense, 
for  now  there  will  be  a  general  shift  toward 
the  red  end  of  the  spectrum.  A  sufficiently 
clear  illustration,  by  analogy,  is  furnished 
by  the  waves  of  sound.  We  know  that 
low-pitched  sounds  are  produced  by  long 
waves,  and  high-pitched  ones  by  short  waves ; 
then  if  the  source  of  the  sound,  such  as  a 
locomotive  whistle,  rapidly  approaches  the 
ear  the  waves  are  crowded  together,  or 
shifted  as  a  whole  toward  the  short  end  of 
the  gamut,  whereupon  the  sound  rises  to  a 


THe  Stars  229 

shrill  scream.  If,  on  the  contrary,  the 
source  of  sound  is  retreating,  the  shift  is  in 
the  other  direction,  and  the  sound  drops 
to  a  lower  pitch. 

This  is  precisely  what  happens  in  the 
spectrum  of  a  star  which  is  either  approach- 
ing or  receding  from  the  eye.  If  it  is  ap- 
proaching, the  Fraunhofer  lines  are  seen 
shifted  out  of  their  normal  position  toward 
the  blue,  and  if  it  is  receding  they  are  shifted 
toward  the  red.  The  amount  of  shifting 
will  depend  upon  the  speed  of  the  star's 
motion.  If  that  motion  is  across  the  line 
of  sight  there  will  be  no  shifting,  because 
then  the  source  of  light  is  neither  approaching 
nor  receding.  Now  take  the  case  of  a  binary 
star  whose  components  are  too  close  to  be 
separated  by  a  telescope.  If  they  happen 
to  be  revolving  round  their  common  centre 
in  a  plane  nearly  coinciding  with  the  line 
of  sight  from  the  earth,  one  of  them  must 
be  approaching  the  eye  at  the  same  time  that 
the  other  is  receding  from  it,  and  the  con- 
sequence is  that  the  spectral  lines  of  the  first 
will  be  shifted  toward  the  blue,  while  those 
of  the  second  are  shifted  toward  the  red. 
The  colours  of  the  two  intermingled  spectra 
blend  into  each  other  too  gradually  to 


230  The  Fixed  Stars 

enable  this  effect  to  be  detected  by  their 
means,  but  the  Fraunhofer  lines  are  sharply 
defined,  and  in  them  the  shift  is  clearly  seen ; 
and  since  there  is  a  simultaneous  shifting 
in  opposite  directions  the  lines  appear  split. 
But  when  the  two  stars  are  in  that  part 
of  their  orbit  where  their  common  motion  is 
across  the  line  of  sight  the  lines  close  up 
again,  because  then  there  is  no  shift.  This 
phenomenon  is  beautifully  exhibited  by  one 
of  the  first  spectroscopic  binaries  to  be  dis- 
covered, Beta  Aurigae.  In  1889,  Prof.  E. 
C.  Pickering  noticed  that  the  spectral  lines 
of  this  star  appeared  split  every  second  night, 
from  which  he  inferred  that  it  consisted  of 
two  stars  revolving  round  a  common  centre 
in  a  period  of  four  days. 

This  spectroscopic  method  has  been  applied 
to  determine  the  speed  with  which  certain 
single  stars  are  approaching  or  receding 
from  the  solar  system.  It  has  also  served 
to  show,  what  we  have  before  remarked, 
that  the  inner  parts  of.  Saturn's  rings  travel 
faster  than  the  outer  parts.  Moreover,  it 
has  been  used  in  measuring  the  rate  of  the 
sun's  rotation  on  its  axis,  for  it  is  plain  that 
one  edge  of  the  sun  approaches  us  while 
the  opposite  edge  is  receding.  Even  the 


The  Stars  231 

effect  of  the  rotation  of  Jupiter  has  been 
revealed  in  this  way,  and  the  same  method 
will  probably  settle  the  question  whether 
Venus  rotates  rapidly,  or  keeps  the  same  face 
always  toward  the  sun. 

Not  only  do  many  stars  revolve  in  orbits 
about  near-by  companions,  but  all  the  stars, 
without  exception,  are  independently  in 
motion.  They  appear  to  be  travelling 
through  space  in  many  different  directions, 
each  following  its  own  chosen  way  without 
regard  to  the  others,  and  each  moving  at  its 
own  gait.  These  movements  of  the  stars 
are  called  proper  motions.  The  directio: 
of  the  sun's  proper  motion  is,  roughly  speak- 
ing, northward,  and  it  travels  at  the  rate  of 
twelve  or  fourteen  miles  per  second,  carry- 
ing the  earth  and  the  other  planets  along 
with  it.  Some  stars  have  a  much  greater 
speed  than  the  sun,  and  some  a  less  speed. 
As  we  have  said,  these  motions  are  in  many 
different  directions,  and  no  attempt  to 
discover  any  common  law  underlying  them 
has  been  entirely  successful,  although  it  has 
been  found  that  in  some  parts  of  the  sky 
a  certain  number  of  stars  appear  to  be 
travelling  along  nearly  parallel  paths,  like 
flocks  of  migrating  birds.  In  recent  years 


232  The  Fixed  Stars 

some  indications  have  been  found  of  the 
possible  existence  of  two  great  general  currents 
of  movement,  almost  directly  opposed  to 
each  other,  part  of  the  stars  following  one 
current  and  part  the  other.  But  no  indica- 
tion has  been  discovered  of  the  existence 
of  any  common  centre  of  motion.  Several 
relatively  near-by  stars  appear  to  be  moving 
in  the  same  direction  as  the  sun.  Stars 
that  are  closely  grouped  together,  like  the 
cluster  of  the  Pleiades,  seem  to  share  a 
common  motion  of  translation  through 
space.  We  have  already  remarked  that 
when  stars  are  found  to  be  moving  toward 
or  away  from  the  sun,  spectroscopic  obser- 
vation of  the  shifting  of  their  lines  gives  a 
means  of  calculating  their  velocity.  In 
other  cases,  the  velocity  across  the  line  of 
sight  can  be  calculated  if  we  know  the 
distance  of  the  stars  concerned.  One  in- 
teresting result  of  the  fact  that  the  earth 
goes  along  with  the  sun  in  its  flight  is  that 
the  orbit  of  the  earth  cannot  be  a  closed 
curve,  but  must  have  the  form  of  a  spiral 
in  space.  In  consequence  of  this  we  are 
continually  advancing,  at  the  rate  of  at 
least  400,000,000  miles  per  year,  toward  the 
northern  quarter  of  the  sky.  The  path 


The  Stars  233 

pursued  by  the  sun  appears  to  be  straight, 
although  it  may,  in  fact,  be  a  curve  so  large 
that  we  are  unable  in  the  course  of  a  lifetime, 
or  many  lifetimes,  to  detect  its  departure 
from  a  direct  line.  At  any  rate  we  know 
that,  as  the  earth  accompanies  the  sun, 
we  are  continually  moving  into  new  regions 
of  space. 

It  has  been  stated  that  many  millions  of 
stars  are  visible  with  telescopes — perhaps 
a  hundred  millions,  or  even  more.  The 
great  majority  of  these  are  found  in  a  broad 
irregular  band,  extending  entirely  round  the 
sky,  and  called  the  Milky  Way,  or  the 
Galaxy.  To  the  naked  eye  the  Milky  Way 
appears  as  a  softly  shining  baldric  encircling 
the  heavens,  but  the  telescope  shows  that 
it  consists  of  multitudes  of  faint  stars, 
whose  minuteness  is  probably  mainly  due 
to  the  immensity  of  their  distance,  although 
it  may  be  partly  a  result  of  their  relative 
lack  of  actual  size,  or  luminosity.  In  many 
parts  of  the  Milky  Way  the  stars  appear 
so  crowded  that  they  present  the  appearance 
of  sparkling  clouds.  The  photographs  of 
these  aggregations  of  stars  in  the  Milky 
Way,  made  by  Barnard,  are  marvellous 
beyond  description.  In  the  Milky  Way,  and 


234  THe  Fixed  Stars 

sometimes  outside  it,  there  exist  globular 
star-clusters,  in  which  the  stars  seem  so 
crowded  toward  the  centre  that  it  is  im- 
possible to  separate  them  with  a  telescope, 
and  the  effect  is  that  of  a  glistering  ball 
made  up  of  thousands  of  silvery  particles, 
like  a  heap  of  microscopic  thermometer 
bulbs  in  the  sunshine.  A  famous  cluster 
of  this  kind  is  found  in  the  constellation 
Hercules. 

The  Milky  Way  evidently  has  the  form 
of  a  vast  wreath,  made  up  of  many  interlaced 
branches,  some  of  which  extend  considerably 
beyond  its  mean  borders.  Within,  this 
starry  wreath  space  is  relatively  empty  of 
stars,  although  some  thousands  do  exist 
there,  of  which  the  sun  is  one.  We  are  at 
present  situated  not  very  far  from  the  centre 
of  the  opening  within  the  ring  or  wreath,  but 
the  proper  motion  of  the  sun  is  carrying  us 
across  this  comparatively  open  space,  and  in 
the  course  of  time,  if  the  direction  of  our 
motion  does  not  change,  we  shall  arrive  at 
a  point  not  far  from  its  northern  border. 
The  Milky  Way  probably  indicates  the 
general  plan  on  which  the  visible  universe 
is  constructed,  or  what  has  been  called  the 
architecture  of  the  heavens,  but  we  still 


The  Stars  235 

know  too  little  of  this  plan  to  be  able  to  say 
exactly  what  it  is. 

The  number  of  stars  in  existence  at  any 
time  varies  to  a  slight  degree,  for  occasion- 
ally a  star  disappears,  or  a  new  one  makes 
its  appearance.  These,  however,  are  rare 
phenomena,  and  new  stars  usually  disappear 
or  fade  away  after  a  short  time,  for  which 
reason  they  are  often  called  temporary  stars. 
The  greatest  of  these  phenomena  ever  beheld 
was  Tycho  Brahe's  star,  which  suddenly 
burst  into  view  in  the  constellation  Cassiopeia 
in  1572,  and  disappeared  after  a  couple  of 
years,  although  at  first  it  was  the  brightest 
star  in  the  heavens.  Another  temporary 
star,  nearly  as  brilliant,  appeared  in  the 
constellation  Perseus,  in  1901,  and  this,  as 
it  faded,  gradually  turned  into  a  nebula, 
or  a  star  surrounded  by  a  nebula.  It  is 
generally  thought  that  outbursts  of  this 
kind  are  caused  by  the  collision  of  two  or 
more  massive  bodies,  t  which  were  invisible 
before  their  disastrous  encounter  in  space. 
The  heat  developed  by  such  a  collision  would 
be  sufficient  to  vaporise  them,  and  thus  to 
produce  the  appearance  of  a  new  blazing 
star.  It  is  possible  that  space  contains  an 
enormous  number  of  great  obscure  bodies, 


236  TKe  Fixed  Stars 

—extinguished  suns,  perhaps — which  are 
moving  in  all  directions  as  rapidly  as  the 
visible  stars. 

2.  The  Nebulae.  These  objects,  which  get 
their  name  from  their  cloud- like  appearance, 
are  among  the  most  puzzling  phenomena  of 
the  heavens,  although  they  seem  to  suggest 
a  means  of  explaining  the  origin  of  stars. 
Many  thousands  of  nebulae  are  known,  but 
there  are  only  two  or  three  bright  enough  to 
be  visible  to  the  naked  eye.  One  of  these 
is  in  the  " sword"  of  the  imaginary  giant 
figure  marking  the  constellation  Orion,  and 
another  is  in  the  constellation  Andromeda. 
They  look  to  the  unaided  eye  like  misty 
specks,  and  require  considerable  attention 
to  be  seen  at  all.  But  in  telescopes  their 
appearance  is  marvellous.  The  Orion  nebula 
is  a  broad,  irregular  cloud,  with  many  brighter 
points,  and  a  considerable  number  of  stars 
intermingled  with  it,  while  the  Andromeda 
nebula  has  a  long  spindle  shape,  with  a 
brighter  spot  in  the  centre.  It  is  covered 
and  surrounded  with  multitudes  of  faint 
stars.  It  was  only  after  astronomical  pho- 
tography had  been  perfected  that  the  real 
shapes  of  the  nebulae  were  clearly  revealed. 
Thousands  of  nebulae  have  been  discovered 


TKe  Nebulae  237 

by  photography,  which  are  barely  if  at 
all  visible  to  the  eye,  even  when  aided 
by  powerful  telescopes.  This  arises  from 
the  fact  that  the  sensitive  photographic 
plate  accumulates  the  impression  that  the 
light  makes  upon  it,  showing  more  and  more 
the  longer  it  is  exposed.  Plates  placed  in 
the  focus  of  telescopes,  arranged  to  utilise 
specially  the  ''photographic  rays,"  are  often 
exposed  for  many  hours  on  end  in  order  to 
picture  faint  nebulas  and  faint  stars,  so 
that  they  reveal  things  that  the  eye,  which 
sees  all  it  can  see  at  a  glance,  is  unable 
to  perceive. 

Nebulas  are  generally  divided  into  two 
classes — the ' '  white ' '  nebulae  and  the ' '  green ' ' 
nebulas.  The  first,  of  which  the  Andromeda 
nebula  is  a  striking  example,  give  a  con- 
tinuous spectrum  without  dark  lines,  as  if 
they  consisted  either  of  gas  under  higfi 
pressure,  or  of  something  in  a  solid  or  liquid 
state.  The  second,  conspicuously  repre- 
sented by  the  Orion  nebula,  give  a  spectrum 
consisting  of  a  few  bright  lines,  characteristic 
of  such  gases  as  hydrogen  and  helium, 
together  with  other  substances  not  yet 
recognised.  But  there  is  no  continuous 
spectrum  like  that  shown  by  the  white  nebulas, 


238  TKe  Fixed  Stars 

from  which  it  is  inferred  that  the  green 
nebulae,  at  least,  are  wholly  gaseous  in 
their  constitution.  The  precise  constitution 
of  the  white  nebulae  remains  to  be  determined. 
It  is  only  in  relatively  recent  years  that 
the  fact  has  become  known  that  the  majority 
of  nebulae  have  a  spiral  form.  There  is 
almost  invariably  a  central  condensed  mass 
from  which  great  spiral  arms  wind  away  on 
all  sides,  giving  to  many  of  them  the  ap- 
pearance of  spinning  pin- wheels,  flinging 
off  streams  of  fire  and  sparks  on  all  sides. 
The  spirals  look  as  if  they  were  gaseous, 
but  along  and  in  them  are  arrayed  many 
condensed  knots,  and  frequently  curving 
rows  of  faint  stars  are  seen  apparently  in 
continuation  of  the  nebulous  spirals.  The 
suggestion  conveyed  is  that  the  stars  have 
been  formed  by  condensation  from  the  spirals. 
These  nebulae  generally  give  the  spectra 
of  the  white  class,  but  there  are  also  some- 
times seen  bright  lines  due  to  glowing  gases. 
The  Andromeda  nebula  is  sometimes  de- 
scribed as  spiral,  but  its  aspect  is  rather 
that  of  a  great  central  mass  surrounded  with 
immense  elliptical  rings,  some  of  which  have 
broken  up  and  are  condensing  into  separate 
masses.  The  Orion  nebula  is  a  chaotic 


THe  Nebulae  239 

cloud,  filled  with  partial  vacancies  and  ribbed 
with  many  curving,  wave-like  forms. 

There  are  other  nebulae  which  have  the 
form  of  elliptical  rings,  occasionally  with  one 
or  more  stars  near  the  centre.  A  famous 
example  of  this  kind  is  found  in  the  constel- 
lation Lyra.  Still  others  have  been  compared 
in  shape  to  the  planet  Saturn  with  its  rings, 
and  some  are  altogether  bizarre  in  form, 
occasionally  looking  like  glowing  tresses 
floating  among  the  stars. 

The  apparent  association  of  nebulae  with 
stars  led  to  the  so-called  nebular  hypothesis, 
according  to  which  stars  are  formed,  as 
already  suggested,  by  the  condensation  of 
nebulous  matter.  In  the  celebrated  form 
which  Laplace  gave  to  this  hypothesis,  it  was 
concerned  specially  with  the  origin  of  our  solar 
system.  He  assumed  that  the  sun  was  once 
enormously  expanded,  in  a  nebulous  state,  or 
surrounded  with  a  nebulous  cloud,  and  that 
as  it  contracted  rings  were  left  off  around 
the  periphery  of  the  vast  rotating  mass. 
These  rings  subsequently  breaking  and  con- 
densing into  globes,  were  supposed  to  have 
given  rise  to  the  planets.  It  is  still  believed 
that  the  sun  and  the  other  stars  may  have 
originated  from  the  condensation  of  nebulae, 


240  The  Fixed  Stars 

but  many  objections  have  been  found  to  the 
form  in  which  Laplace  put  his  hypothesis, 
and  the  discovery  of  the  spiral  nebulae  has 
led  to  other  conjectures  concerning  the  way 
in  which  the  transformation  is  brought 
about.  But  we  have  not  here  the  space  to 
enter  into  this  discussion,  although  it  is  of 
fascinating  interest. 

A  word  more  should  be  said  about  the  use 
of  photography  in  astronomy.  It  is  hardly 
going  too  far  to  aver  that  the  photographic 
plate  has  taken  the  place  of  the  human  retina 
in  recording  celestial  phenomena,  especially 
among  the  stars  and  nebulae.  Not  only 
are  the  forms  of  such  objects  now  exclusively 
recorded  by  photography,  but  the  spectra 
of  all  kinds  of  celestial  objects — sun,  stars, 
nebulae,  etc. — are  photographed  and  afterward 
studied  at  leisure.  In  this  way  many  of  the 
most  important  discoveries  of  recent  years 
have  been  made,  including  those  of  variable 
stars  and  new  stars.  Photographic  charts 
of  the  heavens  exist,  and  by  comparing  these 
with  others  made  later,  changes  which  would 
escape  the  eye  can  be  detected.  Comets 
are  sometimes,  and  new  asteroids  almost 
invariably,  discovered  by  photography.  The 
changes  in  the  spectra  of  comets  and  new 


THe  Constellations  241 

< 

stars  are  thus  recorded  with  an  accuracy  that 
would  be  otherwise  unattainable.  Photo- 
graphs of  the  moon  excel  in  accuracy  all 
that  can  be  done  by  manual  drawing,  and 
while  photographs  of  the  planets  still  fail 
to  show  many  of  the  fine  details  visible  with 
telescopes,  continual  improvements  are  being 
made.  Many  of  the  great  telescopes  now 
in  use  or  in  course  of  construction  are 
intended  specially  for  photographic  work. 

3.  The  Constellations.  The  division  of 
the  stars  into  constellations  constitutes  the 
uranography  or  the  "geography  of  the 
heavens . ' '  The  ma j ority  of  the  constellations 
are  very  ancient,  and  their  precise  origin  is 
unknown,  but  those  which  are  invisible 
from  the  northern  hemisphere  have  all  been 
named  since  the  great  exploring  expeditions 
to  the  south  seas.  There  are  more  than 
sixty  constellations  now  generally  recognised. 
Twelve  of  these  belong  to  the  zodiac,  and 
bear  the  same  names  as  the  zodiacal  signs, 
although  the  precession  of  the  equinoxes 
has  drifted  them  out  of  their  original  relation 
to  the  signs.  Many  of  the  constellations 
are  memorials  of  prehistoric  myths,  and  a 
large  number  are  connected  with  the  story 
of  the  Argonautic  expedition  and  with 

16 


242  The  Fixed  Stars 

other  famous  Greek  legends.  Thus  the 
constellations  form  a  pictorial  scroll  of 
legendary  history  and  mythology,  and  possess 
a  deep  interest  independent  of  the  science 
of  astronomy.  For  their  history  and  for 
the  legends  connected  with  them,  the  reader 
who  desires  a  not  too  detailed  resume,  may 
consult  Astronomy  with  the  Naked  Eye,  and 
for  guidance  in  finding  the  constellations, 
Astronomy  with  an  Opera-glass,  or  Round 
the  Year  with  the  Stars.  The  quickest  way  to 
learn  the  constellations  is  to  engage  the  aid 
of  some  one  who  knows  them  already,  and 
can  point  them  out  in  the  sky.  The  next 
best  way  is  to  use  star  charts,  or  a  star -finder 
or  planisphere. 

A  considerable  number  of  the  brighter  and 
more  important  stars  are  known  by  indi- 
vidual names,  such  as  Sirius,  Canopus, 
Achernar,  Arcturus,  Vega,  Rigel,  Betelguese, 
Procyon,  Spica,  Aldebaran,  Regulus,  Altair, 
and  Fomalhaut.  Astronomers  usually  desig- 
nate the  principal  stars  of  each  constella- 
tion by  the  letters  of  the  Greek  alphabet, 
a,  P,  T,  etc.,  the  brightest  star  in  the  con- 
stellation bearing  the  name  of  the  first 
letter,  the  next  brightest  that  of  the  second 
letter,  and  so  on. 


THe  Constellations  243 

The  constellations  are  very  irregular  in 
outline,  and  their  borders  are  only  fixed 
with  sufficient  definiteness  to  avoid  the 
inclusion  of  stars  catalogued  as  belonging 
to  one,  within  the  limits  of  another.  In  all 
cases  the  names  come  from  some  fancied 
resemblance  of  the  figures  formed  by  the 
principal  stars  of  the  constellation  to  a  man, 
woman,  animal,  or  other  object.  In  only 
a  few  cases  are  these  resemblances  very 
striking. 

The  most  useful  constellations  for  the 
beginner  are  those  surrounding  the  north 
celestial  pole,  and  we  give  a  little  circular 
chart  showing  their  characteristic  stars. 
The  names  of  the  months  running  round  the 
circle  indicate  the  times  of  the  year  when 
these  constellations  are  to  be  seen  on  or 
near  the  meridian  in  the  north.  Turn  the 
chart  so  that  the  particular  month  is  at  the 
bottom,  and  suppose  yourself  to  be  facing 
northward.  The  hour  when  the  observation 
is  supposed  to  be  made  is,  in  every  case, 
about  9  o'clock  in  the  evening,  and  the  date 
is  about  the  first  of  the  month.  The  top  of 
the  chart  represents  the  sky  a  little  below 
the  zenith  in  the  north,  and  the  bottom 
represents  the  horizon  in  the  north. 


244 


TTHe  Fixed  Stars 


The  apparent  yearly  revolution  of  the 
heavens,  resulting  from  the  motion  of  the 
earth  in  its  orbit,  causes  the  constellations 
to  move  westward  in  a  circle  round  the  pole, 


Fig.  18. 


North  Circumpolar  Stars. 


at  the  rate  of  about  30°  per  month.  But  the 
daily  rotation  of  the  earth  on  its  axis  causes 
a  similar  westward  motion  of  the  heavens, 
at  the  rate  of  about  30°  for  every  two  hours. 
From  this  it  results  that  on  the  same  night, 


THe  Constellations 


2*5 


after  an  interval  of  two  hours,  you  will  see  the 
constellations  occupying  the  place  that  they 
will  have,  at  the  original  hour  of  observation, 
one  month  later.  Thus,  if  you  observe  their 


Fig.  ip.     Key  to  North  Circumpolar  Stars. 

positions  at  9  P.M.  on  the  first  of  January,  and 
then  turn  the  chart  so  as  to  bring  February  at 
the  bottom,  you  will  see  the  constellations 
around  the  north  pole  of  the  heavens  placed  as 
they  will  be  at  1 1  P.M.  on  the  first  of  January. 


246  The  Fixed  Stars 

Only  the  conspicuous  stars  have  been 
represented  in  the  chart,  just  enough  being 
included  to  enable  the  learner  to  recognise 
the  constellations  by  their  characteristic 
star  groups,  from  which  they  have  received 
their  names.  The  chart  extends  to  a  dis- 
tance of  40°  from  the  pole,  so  that,  for 
observers  situated  in  the  mean  latitude  of 
the  United  States,  none  of  the  constellations 
represented  ever  descends  below  the  horizon, 
those  that  are  at  the  border  of  the  chart 
just  skimming  the  horizon  when  they  are 
below  the  pole. 

On  the  key  to  the  chart  the  Greek-letter 
names  of  the  principal  stars  have  been  at- 
tached, but  some  of  them  have  other  names 
which  are  more  picturesque.  These  are 
as  follows:  In  Ursa  Major  (the  Great  Bear, 
which  includes  the  Great  Dipper),  a  is 
called  Dubhe,  g  Merak,  r  Phaed,  8  Megrez, 
8  Alioth,  £  Mizar,  and  TQ  Benetnash.  The 
little  star  close  by  Mizar  is  Alcor.  In 
Cassiopeia,  a  is  called  Schedar,  P  Caph, 
and  8  Ruchbar.  In  Ursa  Minor,  the  Little 
Bear,  a  is  called  Polaris,  or  the  North  Star, 
and  P  Kochab.  In  Draco,  « is  called  Thuban, 
and  T  Eltanin.  In  Cepheus,  a  is  called 
Alderamin,  and  P  Alfirk.  These  names  are 


XKe  Constellations  247 

nearly  all  of  Arabic  origin.  It  will  be  ob- 
served that  Merak  and  Dubhe  are  the 
famous  "Pointers,"  which  serve  to  indicate 
the  position  of  the  North  Star,  while  Thuban 
is  the  "star  of  the  pyramid,"  before  men- 
tioned. The  north  celestial  pole  is  situated 
almost  exactly  on  a  straight  line  drawn  from 
Mizar  through  the  North  Star  to  Ruchbar, 
and  a  little  more  than  a  degree  from  the 
North  Star  in  the  direction  of  Ruchbar. 
This  furnishes  a  ready  means  for  ascertaining 
the  position  of  the  meridian.  For  instance, 
about  the  middle  of  October,  Mizar  is  very 
close  to  the  meridian  below  the  pole,  and 
Ruchbar  equally  close  to  it  above  the  pole, 
and  then,  since  the  North  Star  is  in  line  with 
these  two,  it  also  must  be  practically  on 
the  meridian,  and  its  direction  indicates 
very  nearly  true  north.  The  same  method 
is  applicable  whenever,  at  any  other  time 
of  the  year  or  of  the  night,  Mizar  and 
Ruchbar  are  observed  to  lie  upon  a  vertical 
line,  no  matter  which  is  above  and  which 
below.  It  is  also  possible  to  make  a  very 
good  guess  at  the  time  of  night  by  knowing 
the  varying  position  of  the  line  joining 
these  stars. 

The  star  Caph  is  an  important  landmark 


248  THe  Fixed  Stars 

because  it  lies  almost  on  the  great  circle 
of  the  equinoctial  colure,  which  passes  through 
the  vernal  and  autumnal  equinoxes. 

On  the  key,  the  location  of  the  North  Pole 
of  the  Ecliptic  is  shown,  and  the  greater 
part  of  the  circle  described  by  the  north 
celestial  pole  in  the  period  of  25,800  years. 

While  the  reader  who  wishes  to  pursue 
the  study  of  the  constellations  in  detail 
must  be  referred  to  some  of  the  works 
before  mentioned,  or  others  of  like  character, 
it  is  possible  here  to  aid  him  in  making  a 
preliminary  acquaintance  with  other  con- 
stellations beside  those  included  in  our  little 
chart,  by  taking  each  of  the  months  in  turn, 
and  describing  the  constellations  which  he 
will  see  on  or  near  the  meridian  south  of  the 
border  of  the  chart  at  the  same  time  that 
the  polar  constellations  corresponding  to 
the  month  selected  are  on  or  near  the  me- 
ridian in  the  north.  Thus,  at  9  P.M.  about 
the  first  of  January,  the  constellation  Per- 
seus, lying  in  a  rich  part  of  the  Milky  Way, 
is  nearly  overhead  and  directly  south  of  the 
North  Star.  This  constellation  is  marked  by 
a  curved  row  of  stars,  the  brightest  of  which, 
of  the  second  magnitude,  is  Algenib,  or  « 
Persei.  A  few  degrees  south-west  of  Algenib 


THe  Constellations  249 

is  the  wonderful  variable  Algol.  East  of 
Perseus  is  seen  the  very  brilliant  white  star 
Capella  in  the  constellation  Auriga.  This 
is  one  of  the  brightest  stars  in  the  sky. 
Almost  directly  south  of  Perseus,  the  eye 
will  be  caught  by  the  glimmering  cluster 
of  the  Pleiades  in  the  constellation  Taurus. 
A  short  distance  south-east  of  the  Pleiades 
is  the  group  of  the  Hyades  in  Taurus,  shaped 
like  the  letter  V,  with  the  beautiful  reddish 
star  Aldebaran  in  the  upper  end  of  the 
southern  branch  of  the  letter.  The  ecliptic 
runs  between  the  Pleiades  and  the  Hyades. 
Still  lower  in  the  south  will  be  seen  a  part 
of  the  long- winding  constellation  Eridanus, 
the  River  Po.  Its  stars  are  not  bright  but 
they  appear  in  significant  rows  and  streams. 
About  the  first  of  February  the  constel- 
lation Auriga  is  on  the  meridian  not  far  from 
overhead,  Capella  lying  toward  the  west. 
Directly  under  Auriga,  two  rather  conspicu- 
ous stars  mark  the  tips  of  the  horns  of 
Taurus,  imagined  as  a  gigantic  bull,  and 
south  of  these,  with  its  centre  on  the  equa- 
tor, scintillates  the  magnificent  constellation 
Orion,  the  most  splendid  in  all  the  sky, 
with  two  great  first-magnitude  stars,  one, 
in  the  shoulder  of  the  imaginary  giant, 


250  TKe  Fixed  Stars 

of  an  orange  hue,  called  Betelguese,  and  the 
other  in  the  foot,  of  a  blue- white  radiance, 
called  Rigel.  Between  these  is  stretched 
the  straight  line  of  the  "belt,"  consisting 
of  three  beautiful  second-magnitude  stars, 
about  a  degree  and  a  half  apart.  Their 
names,  beginning  with  the  western  one, 
are  Mintaka,  Alnilam,  and  Alnitah.  Di- 
rectly under  the  belt,  in  the  midst  of  a  short 
row  of  faint  stars  called  the  "sword,"  is 
the  great  Orion  nebula.  It  will  be  observed 
that  the  three  stars  of  the  belt  point,  though 
not  exactly,  toward  the  brightest  of  all 
stars,  Sirius,  in  the  constellation  Canis  Major, 
the  Great  Dog,  which  is  seen  advancing  from 
the  east.  Under  Orion  is  a  little  constellation 
named  Lepus,  the  Hare. 

The  first  of  March  the  region  overhead  is 
occupied  by  the  very  faint  constellation 
Lynx.  South  of  it,  and  astride  the  ecliptic, 
appear  the  constellations  Gemini,  the  Twins, 
and  Cancer,  the  Crab.  These,  like  Taurus, 
belong  to  the  zodiac.  The  Twins  are  west- 
ward fron  Cancer,  and  are  marked  by  two 
nearly  equal  stars,  about  five  degrees  apart. 
The  more  westerly  and  northerly  one  is 
Castor  and  the  other  is  Pollux.  Cancer 
is  marked  by  a  small  cluster  of  faint  stars 


The  Constellations  251 

called  Praesepe,  the  Manger  (also  sometimes 
the  Beehive).  Directly  south  of  the  Twins, 
is  the  bright  lone  star  Procyon,  in  the  con- 
stellation Canis  Minor,  the  Little  Dog. 
Sirius  and  the  other  stars  of  Canis  Major, 
which  make  a  striking  figure,  are  seen  south- 
west of  Procyon. 

The  first  of  April  the  zodiac  constellation 
Leo  is  near  the  meridian,  recognisable  by 
a  sickle-shaped  figure  marking  the  head  and 
breast  of  the  imaginary  Lion.  The  bright 
star  at  the  end  of  the  handle  of  the  sickle 
is  Regulus.  Above  Leo,  between  it  and  the 
Great  Dipper,  appears  a  group  of  stars 
belonging  to  the  small  constellation  Leo 
Minor,  the  Little  Lion.  Farther  south  is 
a  winding  ribbon  of  stars  indicating  the 
constellation  Hydra,  the  Water  Serpent. 
Its  chief  star,  Alphard,  of  a  slightly  reddish 
tint,  is  seen  west  of  the  meridian  and  a  few 
degrees  south  of  the  equator. 

At  the  beginning  of  May,  when  the  Great 
Dipper  is  nearly  overhead,  the  small  con- 
stellation Canes  Venatici,  the  Hunting  Dogs, 
is  seen  directly  under  the  handle  of  the 
Dipper,  and  south  of  that  a  cobwebby  spot, 
consisting  of  minute  stars,  indicates  the 
position  of  the  constellation  Coma  Berenices, 


252  The  Fixed  Stars 

Berenice's  Hair.  Still  farther  south,  where 
the  ecliptic  and  the  equator  cross,  at  the 
autumnal  equinox,  is  the  large  constellation 
Virgo,  the  Virgin,  also  one  of  the  zodiacal 
band.  Its  chief  star  Spica,  a  pure  white 
gem,  is  seen  some  20°  east  of  the  meridian. 
Below  and  westward  from  Virgo,  and  south 
of  the  equator,  are  the  constellations  Crater, 
the  Cup,  and  Corvus,  the  Crow.  The  stars 
of  Hydra  continue  to  run  eastward  below 
these  constellations.  The  westernmost,  Cra- 
ter, consists  of  small  stars  forming  a  rude 
semicircle  open  toward  the  east,  while  Corvus, 
which  possesses  brighter  stars,  has  the  form 
of  a  quadrilateral. 

The  first  of  June  the  great  golden  star 
Arcturus,  whose  position  may  be  found  by 
running  the  eye  along  the  curve  of  the  handle 
of  the  Great  Dipper,  and  continuing  onward 
a  distance  equal  to  the  whole  length  of  the 
Dipper,  is  seen  approaching  the  meridian 
from  the  east  and  high  overhead.  This 
superb  star  is  the  leader  of  the  constellation 
Bootes,  the  Bear-Driver.  Spica  in  Virgo  is 
now  a  little  west  of  the  meridian. 

The  first  of  July,  when  the  centre  of  Draco 
is  on  the  meridian  north  of  the  zenith,  the 
exquisite  circlet  of  stars  called  Corona 


THe  Constellations  253 

Borealis,  the  Northern  Crown,  is  nearly 
overhead.  A  short  distance  north-east  of 
it  appears  a  double-quadrilateral  figure, 
marking  out  the  constellation  Hercules, 
while  directly  south  of  the  Crown  a  crooked 
line  of  stars  trending  eastward  indicates  the 
constellation  Serpens,  the  Serpent.  South- 
west of  Serpens,  two  widely  separated  but 
nearly  equal  stars  of  the  second  magnitude 
distinguish  the  zodiacal  constellation  Libra, 
the  Balance;  while  lower  down  toward 
the  south-east  appears  the  brilliant  red  star 
Antares,  in  the  constellation  Scorpio,  likewise 
belonging  to  the  zodiac. 

On  the  first  of  August  the  head  of  Draco 
is  on  the  meridian  near  the  zenith,  and  south 
of  it  is  seen  Hercules,  toward  the  west,  and 
the  exceedingly  brilliant  star  Vega,  in  the 
constellation  Lyra,  the  Lyre,  toward  the 
east.  Vega,  or  Alpha  Lyras,  has  few  rivals 
for  beauty.  Its  light  has  a  decided  bluish- 
white  tone,  which  is  greatly  accentuated 
when  it  is  viewed  with  a  telescope.  South 
of  Hercules  two  or  three  rows  of  rather 
large,  widely  separated  stars  mark  the 
constellation  Ophiuchus,  the  Serpent-Bearer. 
This  extends  across  the  equator.  Below  it, 
in  a  rich  part  of  the  Milky  Way,  is  Scorpio, 


254  THe  Fixed  Stars 

whose  winding  line,  beginning  with  Antares 
west  of  the  meridian,  terminates  a  consider- 
able distance  east  of  the  meridian  in  a  pair  of 
stars  representing  the  uplifted  sting  of  the 
imaginary  monster. 

The  first  of  September  the  Milky  Way  runs 
directly  overhead,  and  in  the  midst  of  it 
shines  the  large  and  striking  figure  called 
the  Northern  Cross,  in  the  constellation 
Cygnus,  the  Swan.  The  bright  star  at  the 
head  of  the  Cross  is  named  Denib.  Below 
the  Cross  and  in  the  eastern  edge  of  the 
Milky  Way  is  the  constellation  Aquila,  the 
Eagle,  marked  by  a  bright  star,  Altair, 
with  a  smaller  one  on  each  side  and  not 
far  away.  Low  in  the  south,  a  little  west 
of  the  meridian  and  partly  immersed  in 
the  brightest  portion  of  the  Milky  Way, 
is  the  zodiacal  constellation  Sagittarius, 
the  Archer.  It  is  distinguished  by  a  group 
of  stars  several  of  which  form  the  figure 
of  the  upturned  bowl  of  a  dipper,  some- 
times called  the  Milk  Dipper.  East  of 
Cygnus  and  Aquila  a  diamond-shaped  figure 
marks  the  small  constellation  Delphinus, 
the  Dolphin. 

At  the  opening  of  October,  when  Denib 
is  near  the  meridian,  the  sky  directly  in  the 


THe  Constellations  255 

south  is  not  very  brilliant.  Low  down, 
south  of  the  equator,  is  seen  the  zodiacal 
constellation  Capricornus,  the  Goat,  with 
a  noticeable  pair  of  stars  in  the  head  of  the 
imaginary  animal. 

On  the  first  of  November,  when  Cassiopeia 
is  approaching  the  meridian  overhead,  the 
Great  Square,  in  the  constellation  Pegasus, 
is  on  the  meridian  south  of  the  zenith, 
while  south-west  of  Pegasus  the  zodiacal 
constellation  Aquarius,  the  Water-Bearer, 
appears  on  the  ecliptic.  A  curious  scrawling 
Y-shaped  figure  in  the  upper  part  of  Aquarius 
serves  as  a  mark  to  identify  the  constellation. 
Thirty  degrees  south  of  this  shines  the  bright 
star  Fomalhaut,  in  the  constellation  Piscis 
Australis,  the  Southern  Fish.  The  two 
stars  forming  the  eastern  side  of  the  Great 
Square  of  Pegasus  are  interesting  because, 
like  Caph  in  Cassiopeia,  they  lie  close  to  the 
line  of  the  equinoctial  colure.  The  northern 
one  is  called  Alpheratz  and  the  southern 
Gamma  Pegasi.  Alpheratz  is  a  star  claimed 
by  two  constellations,  since  it  not  only  marks 
one  corner  of  the  square  of  Pegasus,  but 
it  also  serves  to  indicate  the  head  of  the 
maiden  in  the  celebrated  constellation  of 
Andromeda. 


The  Fixed  Stars 


The  first  of  December,  Andromeda  is  seen 
nearly  overhead,  south  of  Cassiopeia.  The 
constellation  is  marked  by  a  row  of  three 
second-magnitude  stars,  beginning  on  the 
east  with  Alpheratz  and  terminating  near 
Perseus  with  Almaack.  The  central  star  is 
named  Mirach.  A  few  degrees  north-west 
of  Mirach  glimmers  the  great  Andromeda 
nebula.  Below  Andromeda,  west  of  the 
meridian,  appears  the  zodiacal  constellation 
Aries,  the  Ram,  indicated  by  a  group  of 
three  stars,  forming  a  triangle,  the  brightest 
of  which  is  called  Hamal.  South-westerly 
from  Aries  is  the  zodiacal  constellation  Pisces, 
the  Fishes,  which  consists  mainly  of  faint 
stars  arranged  in  pairs  and  running  far  toward 
the  west  along  the  course  of  the  ecliptic, 
which  crosses  the  equator  at  the  vernal 
equinox,  near  the  western  end  of  the  con- 
stellation. South  of  Pisces  and  Aries  is 
the  broad  constellation  Cetus,  the  Whale, 
marked  by  a  number  of  large  quadrilateral 
and  pentagonal  figures,  formed  by  its  stars. 
Near  the  centre  of  this  constellation,  but 
not  ordinarily  visible  to  the  naked  eye,  is 
the  celebrated  variable  Mir  a,  also  known  as 
Omicron  Ceti. 

With  a  little  application  any  person  can 


TKe  Constellations  257 

learn  to  recognise  these  constellations,  even 
with  the  slight  aid  here  offered,  and  if  he 
does,  he  will  find  the  knowledge  thus  acquired 
as  delightful  as  it  is  useful. 


INDEX 


Aberration,  88 
Alpha  Centauri,  145 
Alpha  Draconis,  62 
Altitude,  13, 14 
Andromeda,  nebula  in,  236 
Antares,  24 
Aphelion,  8 

April,  aspect  of  sky  in,  251 
Arcturus,  224 
Asteroids,  the,  187 
Atmosphere,  the,  82 
August,  aspect  of  sky  in, 

253 
Azimuth,  13, 14 

Beta  Aurigae,  230 

Binary  (and  multiple)  stars, 

226 
Binaries,  spectroscopic,  227 

Calendar,  the,  117 

Calorie,    measure   of   sun's 

heat,  129 
Cassiopeia,  22 
Cheops,  pyramid  of,  62 
Chromosphere,  of  sun,  135 
Circles,    vertical,    15;    alti- 
tude, 14;  division  of,  16 
Circumpolar  stars,  224 
Clock,     the     astronomical, 

41,90 

Comets,  201,203 
Constellations,  the,  241 
Corona,  of  the  sun,  135 


Coronium,  152 
Corona  Borealis,  24 

Day,  change  of,  99, 101 
Day  and  night,  96 
December,  aspect  of  sky  in, 

256 

Declination,  3 1 , 35 
Dipper,  the  Great,  21;  the 

Little,  23 

Earth,  description  of  the, 

67 ;  weight  of,  73 
Eclipses,  163 
Ecliptic,  43 
Elements,  chemical,  in  the 

sun,  151 
Equator,  38 
Equinoxes,  45 
Eros,  the  asteroid,  142 

February,  aspect  of  sky  in, 
249 

Galaxy,  the,  233 
Gravitation,  laws  of,  70 
"Greenwich  of  the  Sky," 
32 

Heavens,  apparent  motion 

of,  19 

Helium,  152 

Horizon,  10, 12;  dip  of,  86 
Hour  Circles,  31 


259 


260 


Index 


January,  aspect  of  sky  in, 

248 

July,  aspect  of  sky  in,  252 
June,  aspect  of  sky  in,  252 
Jupiter,  the  planet,  189 

Kepler,  laws  of,  173 

Latitude,     terrestrial,     30; 

celestial,  49,  51 
Light,  pressure  of,  206 
Longitude,    terrestrial,   30; 

celestial,  49,  51 

Magnitudes,  stellar,  220 
March,  aspect  of  sky  in,  250 
Mars,  the  planet,  181 
May,  aspect  of  sky  in,  251 
Mercury,  the  planet,  175 
Meteors,  2 10 
Meteorites,  214 
Milky  Way,  the,  233 
Molecules,    escape    from 
planetary     atmospheres, 

158 
Moon,  description  of,  153; 

how  earth  controls,  75 
Motion,    proper,  of    solar 

system,  232 

Nadir,  1 1 

Nebulae,  236 

Nebular  Hypothesis,  239 

Neptune,  the  planet,  199 

Noon-line,  12 

North  Point,  20 

North  Pole,  27, 37 

North  Star,  20, 27,  61 

November,   aspect  of   sky 

in,  255 
Nutation,  63 

Oblique  sphere,  39 
October,  aspect  of  sky  in, 

254 
Oppositions,  of  Mars,  183 


Orbits  of  planets,  7,  52 
Orion,  nebula  in,  236 

Parallax,  136, 139 
Parallel  sphere,  38 
Perihelion,  8 
Phases  of  the  moon,  1 60 
Photography,  in  astronomy, 

240 

Photometry,  of  stars,  220 
Photosphere  of  sun,  134 
Planets,  description  of  the, 

172 

Polar  circles,  1 1 1 
Precession  of  the  Equinoxes, 

55 

Refraction,  83,  85 
Right  Ascension,  31, 35 
Right  sphere,  39 
Rotation,  effect  of  earth's, 


Saturn,  description  of,  195 
Seasons,     the,     104,     107; 

secular  change  of ,  1 1 5 
September,  aspect  of  sky  in, 

254 

Signs  of  Zodiac,  54 

Spectroscopic  analysis,  145, 
147;  shifting  of  lines,  227 

Sphere,  celestial,  9 

Solstices,  the,  47 

Stars,  the  fixed,  219;  daily 
revolution  of,  25;  how 
to  locate,  29;  luminosity 
of,  222;  spectra  of,  221; 
variable,  225;  temporary, 
235;  double  and  multiple, 
226 

Sun,  the  description  of ,  127 

Sunspots,  131 

Temperature,  of  sun,  130 
Temporary  stars,  235 
Tides,  the,  76,  79 


Index 


261 


Time,  89;  sidereal  and  solar, 

93 

Transits,  of  Venus,  140, 141 
Tropics,  the,  1 1 1 

Universe,  appearance  of,  8 
Uranus,  description  of,  199 
Ursa  Major,  21 


Variable  stars,  225 
Vega,  63 

Venus,  the  planet,  178 
Vernal  Equinox,  32, 42, 46 


Zenith,  10 
Zodiac,  50 


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